--- title: "The Boötes Void: The Biggest, Emptiest Thing in the Universe" description: "Space. The final frontier. For millennia, humanity has looked up into the stars, wondering about the great things beyond our vision and reach. Could it be distant advanced alien civilisations? Other universes? Planets that rain pure 24-carat gold? And then we got there and we realised… space is pretty empty.\n\nCan you imagine nothing? What if there was absolutely nothing around you right now. No computer or phone. No room, no house, no land, no sea, no sky, no sun. Just pure blackness. Absolutely nothing in every direction. An infinite landscape of absolutely nothing. That's what's up in space for the most part—nothing whatsoever. Whilst the introverts among you may think of hiding away in the dark recesses of space as the best thing ever, I can assure you it isn't. An infinite void of nothing, only darkness, isolation and a deafening silence.\n\nWhat's more, there is a place in our universe defined by this very notion of nothing. Far off in the distant reaches of the cosmos lies an area so vast and so empty, it has prompted conspiracy theories about hyper-advanced civilisations, it has inspired stories of sci-fi fantasy and it has even taught us a little about the universe itself.\n\nIt is the universe's desert. A place so vast and empty, all the money on earth converted into pennies wouldn't even fill a fraction of 0.01% of it.\n\nWhat happens when you stare into the abyss and the abyss stares back?\n\nThe Boötes Void.\n\n## What Actually Is a Cosmic Void?\n\nSo, one of the first questions we have to ask ourselves in order to understand the Boötes Void is: what is a void? Scientifically speaking, a void in space is a vast region with a very low cosmic mean density. That's a fancy way of saying there's nothing in it, or rather, very little in it. You see, despite being called a void, the Boötes Void is far from totally empty. A void's definition comes from its low cosmic density relative to the rest of space.\n\nThe Boötes Void, one of the largest in the known universe, is 330 million light years across according to the BBC. It's absolutely enormous. But its size actually gives a good reference point for just how empty it is compared to the rest of space. In an average slice of space that's the same size as the Boötes Void, you would expect to find over 2,000 galaxies in it, meaning that on average there is a galaxy every 165,000 light years or so. In the Boötes Void, there are 60 galaxies. Which means on average, you would only expect to run into another galaxy once every 5.5 million light years.\n\nThere are great expanses of nothing here on earth—deserts and oceans where you can see absolutely nothing in every direction right to the horizon. Well, the number of earths it would take to fit into the Boötes Void is 1.47×1051. That is a quadrillion, quadrillion, quadrillion earths' worth of space. BBC's *Sky at Night* magazine states that a region the volume of the Milky Way could fit into it billions of times over. All with just 60 galaxies in it.\n\nThese galaxies are sometimes known as \"field galaxies\" because they are isolated from the large clusters of galaxies that dominate other regions of the universe. There is something to be said about the idea of being in a galaxy in the middle of an enormous void and the major change in perspective to space that it would cause. If earth was in the middle of the Boötes Void, until we discovered how to map the universe, we would be led to believe that space is almost entirely empty, with our own galaxy being one of the only things in it. If we were in the middle, it's not like we would be able to get our satellites out of the void very easily. We may have discovered and come across a few other galaxies and planets, but certainly not the overwhelming abundance of them in the universe like we come to expect today.\n\nAs astronomer Greg Aldering put it: \"If the Milky Way had been in the centre of the Boötes void, we wouldn't have known there were other galaxies until the 1960s.\"\n\nVoids themselves aren't a new thing, with the first one being discovered in the 19th century. However, nobody at that time could predict the sheer size and scale of universal structures. The Boötes Void is so massive that it takes up around 2% of the diameter of the observable universe.\n\nIt is referred to as the \"Great Void\" as well as \"The Great Nothing\". The name Boötes Void comes from the fact that the void contains the constellation of Boötes, the herdsman who pushes the plough (Ursa Major) around Polaris (the North Star). Poor Boötes, doomed to circle forever in a vast sea of nothing, truly a fate worse than death.\n\nThe void is also surrounded by the Boötes superclusters, galaxy clusters that exist around the edges of the void, among other galaxies and stars. It's also really far away—a full 700 million light years away. So rest assured, humanity won't be visiting any time soon. With current technology, it would take around 13 trillion years to reach it. If you wanted to cross it, it would be another 6.\n\nSo sure, it's big and it's empty, but that poses a question. A void is just cosmic nothing; space is already mostly nothing. How did we find out the Boötes Void even existed in the first place? Well, for that, we need to go back to the 1970s and 1980s and to a man who would change science forever: an American scientist by the name of Robert Kirshner.\n\n## The Discovery of the Boötes Void\n\nIn the 1970s, we were still just dipping our toe into the stars. We had been to the moon, orbited the earth, launched satellites and probes into deep space, but our understanding of space still lacked depth. That is until redshift surveys came along.\n\nRedshift surveys are a type of astronomical survey that are used to measure the distribution of interstellar objects by calculating their \"redshifts\". A redshift occurs when light or other electromagnetic radiation from an object is stretched to longer wavelengths (toward the red part of the light spectrum) as the object moves away from the observer. What is the universe doing at all times? Expanding. So objects are almost always moving further away, stretching that light. We can measure the distance from the earth to the redshifted light of interstellar objects to see how far away they are as well as how quickly they are moving away from us. In effect, this gives us a way to map the observable universe in full 3D.\n\nSo, in the mid-1970s, redshift surveys were starting to become common in our pledge to understand the stars. By the 1980s is when our hero, Robert Kirshner, came in. In 1981, Robert was working alongside his fellow astronomers at the University of Michigan, as well as at the Harvard-Smithsonian Institute for Astrophysics, in order to calculate the redshift of a large number of galaxies in the observable universe. He would go on to become a professor at the institute later in life.\n\nRobert and his colleagues were doing the survey when one day they discovered a huge blank region, bigger than any void that had been seen up to that point, in a portion of the universe around 700 million light years away. In this part of the universe, Kirshner and co expected to see large swathes of redshifted light, and there was in a lot of other areas. But right where we now know the Boötes Void sits, there was simply nothing. Was it a problem with the software? No, the computer was working fine. The group had stumbled across a hitherto undiscovered void—a super void. There were other voids that had been found before this point, as we mentioned redshift surveys go back to the mid-70s, but definitely none this size. It was a real-life marvel, right in our own universe.\n\nBut also, as I mentioned, it wasn't the only one. Yes, the discovery of the Boötes Void led to major breakthroughs in our understanding of the universe, but it technically isn't the biggest, emptiest thing in the universe. It certainly was at the time it was discovered; however, since then more complete maps have been drawn of the observable universe and it turns out the Boötes Void was not the only super void on the block.\n\nThe biggest of them all is known as the KBC void—it is a staggering 2 billion light years across, as opposed to the Boötes Void's comparatively puny 330 million. Other voids over a billion light years across do exist, like the Eridanus super void and the Sloan Great Wall void, leaving the great Boötes Void as something of a historical and statistical afterthought by comparison.\n\nVoids in space have captivated audiences across a wide range of sci-fi media, from *Star Trek* to Peter F. Hamilton's *Commonwealth Saga*. There is something of a call of the void, a deeply human macabre interest in these areas of space that house so little.\n\nWhere is the Boötes Void today? Where it's almost always been and will continue to be for a long time yet—near the Boötes constellation, 700 million light years away. Well, as far as we know anyway. The light that reaches our equipment on earth has been travelling for 700 million years. So any images we do have of the void will be primordial. But this can be useful. We can use the Boötes Void and others like it to understand some of the most pressing questions about our universe, and I've got a pretty good one: why is it there?\n\n## Where Do Voids Come From?\n\n\"That's all well and good,\" you might say, \"but how can we answer the prevailing question of why such a large expanse of space is filled with seemingly nothing at all?\" We all know that space is mostly empty—it's why it's called space. However, this significantly sized chunk of it is distinctly emptier. A drop from several thousand galaxies to 60 is over a 95% decrease in matter in the area. That seems like an anomaly, doesn't it? But the sheer size of the thing and the fact we know there are larger ones defies that very notion. Why is this great void of nothing here?\n\nWell, our understanding of these voids is like a negative photo. They are defined by what they lack. Scientists have created detailed maps of our universe and when putting the pieces together, it looks like a giant spider's web. The vast majority of galaxies in our universe are found on the threads of this web. They're known as \"filaments\". Whilst the empty spaces in the middle are known as voids. Have you ever seen a population density map of Australia? That's what one of these individual voids with galaxies around the edge would look like. Except there are thousands in the observable universe, all linked together.\n\nSo voids are actually relatively common. There's even one near the Milky Way, creatively named the Local Void. They make up 80% of the observable universe and many are millions of light years across. Where galaxy filaments meet at areas of high concentration, it's known as a cluster. We're in one right now. The Milky Way is part of the Virgo cluster, which in turn is part of the Laniakea Supercluster. These filaments and voids are the largest structures in the known universe, and the Boötes Void is simply one of the largest pieces of the puzzle.\n\nOk great, but why does it exist?\n\nThe BBC's *Science Focus* asserts that in order to understand why we have these voids and filaments, we have to go back to where it all began: the Big Bang. When the universe was first formed, the matter contained within it (which encompasses all the mass in the known universe) was densely packed together because the universe was still small. Scientists' theory is that in this densely packed state, quantum fluctuations within the densely packed matter caused some areas of matter to be slightly more densely packed than others. Denser areas began to gather matter quickly, owed to gravity, pulling in other matter from the less densely packed areas. As the universe continued to expand outwards, these denser areas continued to grow and clump together, eventually forming the galaxies we see today. However, as those less densely packed areas also began to expand, they continued to lose matter through gravity to the more densely packed areas. So over an excruciatingly long period of time, there was less and less matter in those areas, thus creating voids; similarly, there was more and more matter in the clusters, which then created the filaments. This led to the formation of the cosmic web as we know it today.\n\nThis makes logical sense from a scientific standpoint. Small fluctuations over billions of years would make quite the difference as the universe continued to expand in that time.\n\nThese individual voids are like bubbles in foam, where smaller bubbles can merge together into larger ones as they expand. Many believe this is how the Boötes Void, alongside the other larger voids in the universe, came to be. These filaments and voids, weaved together like a spider web, have allowed us to map large portions of the observable universe. The clusters of galaxies that make up the filaments give off more energy than the colder, mostly empty voids. This means we can map them when using thermal imaging, which is effectively how we discovered the concept in the first place.\n\nJoseph Clampitt, a post-doctoral researcher at the University of Pennsylvania, explained the phenomenon in a novel way using height as a stand-in for density, quoting here: \"Voids are vast, shallow troughs in the dark matter [distribution], galaxies sit at the centre of narrow, towering peaks. A typical galaxy has the same height and footprint as the Empire State Building [and] a medium-size void would be a 3-foot-deep hole three times the size of Manhattan.\"\n\nSo the Boötes Void is just that—a void. A relic of a time long passed when the universe began, that expanded to become one of the largest things in the observable universe. A large hole in space filled to the brim with nothing at all. Of course, there are a few galaxies, but the Boötes Void is largely a barren wasteland. It conjures up thoughts of what it would be like to exist inside it for even a minute. To look around you and see an endless expanse of darkness in every direction. Not even planets, galaxies or stars. It has a funny allure to it. The void calls for us all.\n\nHowever, there are some that don't believe that the Boötes Void is an afterthought of a tiny fluctuation in energy when the universe began. Instead, they have pioneered a set of theories, some wackier than others, in order to explain why there is a 330 million light year expanse of nothing in the middle of space. Let's explore those, shall we?\n\n## Fringe Theories – The Good, the Bad and the Wacky\n\nAs our understanding of voids has only really grown since the 1980s—which is relatively recent in scientific terms; we went to the moon in the 60s after all—there are a lot of fringe theories with varying degrees of scientific merit to explain phenomena within the Boötes Void. They vary from the relatively scientifically sound all the way into conspiracy theory nonsense.\n\nSo, what do people think the Boötes Void is really? We'll start with the obvious one: a black hole. It would make some scientific sense for the Boötes Void to be a black hole. After all, there seems to be so little there relative to other more densely packed parts of the universe that even light can't seem to pass all the way through the void, indicating that the light could be being affected by some massive interstellar object. We know black holes have strong enough gravity to affect and even trap light. So in theory, it could be a reason some voids exist.\n\nHowever, when we start to think about it as an explanation for the Boötes Void in more depth, it just doesn't add up. The largest black hole ever discovered is TON 618. NASA states it's 66 billion times bigger than the sun. Its event horizon is 0.02 light years across. The Boötes Void is 330 million light years across. This is a whole other level of scale. Simply put, if the Boötes Void was a black hole, we would have much, MUCH bigger problems. Not to mention we'd be able to see massive light distortions around that area of space, and one would expect to see the 60 galaxies in the void affected somehow by a massive black hole. Also, the fact that stars collapse into black holes puts this theory to bed. In order for a star to collapse into a black hole, it needs an extremely large amount of mass. Moreover, given what we know about the cosmic web, where most of the universe's mass is concentrated along dense filaments, it's unlikely that anything with significant mass like a black hole could be present in the Boötes Void. Now I'll grant you, that doesn't make it impossible for there to be an enormous black hole in the Boötes Void that scientists somehow simply haven't found for some reason. After all, there are stars in it, and a large enough one could cause a black hole if it collapsed. However, what we know of the formation of these voids simply makes this hypothesis less likely.\n\nIn a similar capacity, some have theorised that the Boötes Void is a dark nebula, which are molecular clouds so dense that they obscure visible light, masking the light behind them. This too makes some sense, but again there is a size mismatch here. The biggest dark nebula we know of, the Coalsack Nebula, is only around 30 light years across versus the 330 million for the Boötes Void. We also run into the same issues with the 60 galaxies in the void—a dark nebula would obscure them from view. Finally, given that dark nebulae are often found in galaxy structures, and given the relative lack of galaxies in the void, it's highly unlikely one could grow large enough to explain the Boötes Void.\n\nAlright, we've done some proper ones, now let's do the silly stuff. Some have claimed that the Boötes Void actually doesn't exist. That's right—a 330 million light year void in space actually isn't real. What is there, you may ask? Dyson spheres.\n\nA Dyson Sphere is an idea crafted by physicist and mathematician Freeman Dyson in 1960. The idea goes that it could be possible for advanced civilisations to build a superstructure around a star in order to harness as much of its energy as possible. These superstructures are known as Dyson Spheres. I'm sure you can see where this is going. It's asserted that the Boötes Void is not a void at all. It is in fact a hyper-advanced civilisation thousands of years ahead of humans in its development. Not only has this civilisation pioneered interstellar travel and Dyson Sphere building, but mastered it. They mastered it to such an extent that there is now a 330 million light year dark patch in space, as most of the stars have been covered up.\n\nNow we don't need to dive into a complex analysis on this one; we simply have to play a short game of Occam's Razor. What's more likely: a civilisation advanced enough to cover 330 million light years' worth of stars in Dyson Spheres (amounting to billions of stars), or there's a bit of space that's emptier than the rest of space, which is already 99% empty? I rest my case, your honour.\n\nOne of the more minor theories about large voids in general is that they have some kind of special interactions with dark energy and antimatter. Another asserts that large voids contain remnants of \"bubble universes\". However, rather than just for silly goofs or as wild speculation, there is one theory about large voids that may tell us a lot about the fate of the universe itself.\n\nAs we understand it, those voids were formed when all of the mass in the universe was packed tightly together before the Big Bang. When the universe expanded, the minor quantum fluctuations eventually became a web of mass that litters the cosmos, and the spaces between the filaments were the voids. Some scientists theorise that the universe could keep expanding indefinitely, while others think it will reach a certain size and then retreat back in. However, for those who think the universe will continue to expand, it gives us a potential window into the future of the universe itself. The large voids we see in the universe tell us that this is the result of a massive expansion over a very long period of time. If the universe is ever-expanding, however, and will continue to do so in perpetuity, then the distances between matter will continue to grow indefinitely too. After billions or maybe even trillions of years' time, stars will burn out, galaxies will become isolated, and the universe may reach a state of near-total emptiness, not unlike what we see in the Boötes Void today. In that sense, Boötes could be a window into the long-term fate of the cosmos.\n\n## What's So Special About the Boötes Void Anyway?\n\nOk, so we've talked about the Boötes Void. We've established it's likely a semi-common phenomenon that happens throughout the universe. We've learned it's not even the biggest void that we know of in space. So why is it special? Why focus on this specific void if it's no different from the others except it's a little bigger than most of them?\n\nWell, because in a funny way, the Boötes Void is a gift.\n\nYou're right—in the grand scale of the universe, the Boötes Void is like any other. In the cosmic expanse it may not matter, but to us, it's special. The Boötes Void was not just some void we discovered; it is one of the first voids that was ever discovered and certainly the largest at the time. So it's partially significant for that reason, but that's not all.\n\nKirshner's work has since been built upon. Not only can voids potentially indicate a direction of travel for our universe in the future, but they also give us an indication of our past. In studying these structures, astrophysicists today can gaze through a window into what the universe looked like in its earliest moments. BBC's *Science Focus* states that advancements in both telescope and imaging technology have made it possible to create more detailed versions of Kirshner's maps, such as the Dark Energy Survey which has mapped out a quarter of the southern sky and examined around 300 million galaxies. Meanwhile, supercomputers can now create detailed simulations of the universe's expansion. By comparing these maps with the simulations, astronomers can begin to understand how our universe came to look the way it does today.\n\nIn no short order, the Boötes Void altered the way we think about the universe. It has helped make up one small nugget of our understanding of our own universe, which by proxy helps you and I understand our place in it all. So I'm grateful to that 330 million light year void, because without it, we may not understand just how incredibly fortunate we are to be here today. Simply put, the Boötes Void may be a huge expanse of nothing, but it's given us a piece of our everything.\n\n## Key Takeaways\n\n- The Boötes Void is a vast, nearly empty region of space, 330 million light-years across.\n- It contains only 60 galaxies, far fewer than expected for its size.\n- The void was discovered in the 1980s using redshift surveys, which map the universe in 3D.\n- Cosmic voids like Boötes are formed from quantum fluctuations during the Big Bang.\n- The Boötes Void has significantly advanced our understanding of the universe's structure and expansion.\n\n## Frequently Asked Questions\n\n### What is the Boötes Void?\n\nThe Boötes Void is a vast region in space with a very low cosmic mean density, making it one of the largest and emptiest known voids in the universe. It is approximately 330 million light years across and contains only about 60 galaxies.\n\n### How was the Boötes Void discovered?\n\nThe Boötes Void was discovered in the 1980s by Robert Kirshner and his colleagues using redshift surveys. They noticed a large blank region in the universe where they expected to see redshifted light from galaxies.\n\n### What are cosmic voids?\n\nCosmic voids are vast regions in space with a very low cosmic mean density. They are defined by their low density relative to the rest of space and are part of the cosmic web structure of the universe.\n\n### How many galaxies are in the Boötes Void?\n\nThe Boötes Void contains approximately 60 galaxies, which is significantly fewer than the expected 2,000 galaxies in a similarly sized region of space.\n\n### What is the significance of the Boötes Void?\n\nThe Boötes Void is significant because it was one of the first large voids discovered and has contributed to our understanding of the universe's structure and expansion. It provides insights into the early moments of the universe and the potential future of cosmic expansion.\n\n### What are some fringe theories about the Boötes Void?\n\nSome fringe theories suggest that the Boötes Void could be a black hole, a dark nebula, or even a region filled with Dyson spheres built by an advanced civilization. However, these theories are generally considered less likely based on current scientific understanding.\n\n### How does the Boötes Void relate to the cosmic web?\n\nThe Boötes Void is part of the cosmic web structure, where most of the universe's mass is concentrated along dense filaments, and the empty spaces between these filaments are voids. The Boötes Void is one of the largest voids in this web.\n\n### What is the distance to the Boötes Void?\n\nThe Boötes Void is located approximately 700 million light years away from Earth, making it one of the most distant and largest known voids in the observable universe.\n\n### What are redshift surveys and how are they used to study the universe?\n\nRedshift surveys are astronomical surveys that measure the distribution of interstellar objects by calculating their redshifts. This helps in mapping the observable universe in 3D by determining the distance and velocity of objects moving away from Earth.\n\n### What is the potential future of the universe based on the study of voids?\n\nThe study of large voids like the Boötes Void suggests that the universe may continue to expand indefinitely, leading to a state of near-total emptiness over billions or trillions of years. This provides a window into the long-term fate of the cosmos.\n\n## Sources\n\n- [Original Places video: The Boötes Void: The Biggest, Emptiest Thing in the Universe](https://www.youtube.com/watch?v=Z2xGOvUIX68)\n- [Hero image source](https://images.rawpixel.com/editor_1024/czNmcy1wcml2YXRlL3Jhd3BpeGVsX2ltYWdlcy93ZWJzaXRlX2NvbnRlbnQvbHIvdXB3azYxNjkzODU2LXdpa2ltZWRpYS1pbWFnZS1rb3dsbDVsMS5qcGc.jpg) by openverse, cc0.\n\n## Related Coverage" url: https://places.site/article/bootes-void-biggest-emptiest-thing-universe.md canonical: https://places.site/article/bootes-void-biggest-emptiest-thing-universe datePublished: 2026-07-01 dateModified: 2026-07-01 author: - name: Simon Whistler url: https://places.site/author/simon-whistler publisher: Places image: "https://media.places.site/cdn-cgi/image/width=1600,height=900,fit=cover,quality=80,format=auto/articles/Z2xGOvUIX68/hero.jpg" type: NewsArticle contentHash: 9545d13813397831deecc4d0dd9e8a386cb268f1e9c0957e9ade433bd5d7e040 tokens: 6572 summaryUrl: https://places.site/article/bootes-void-biggest-emptiest-thing-universe.md.summary.md --- Space. The final frontier. For millennia, humanity has looked up into the stars, wondering about the great things beyond our vision and reach. Could it be distant advanced alien civilisations? Other universes? Planets that rain pure 24-carat gold? And then we got there and we realised… space is pretty empty. Can you imagine nothing? What if there was absolutely nothing around you right now. No computer or phone. No room, no house, no land, no sea, no sky, no sun. Just pure blackness. Absolutely nothing in every direction. An infinite landscape of absolutely nothing. That's what's up in space for the most part—nothing whatsoever. Whilst the introverts among you may think of hiding away in the dark recesses of space as the best thing ever, I can assure you it isn't. An infinite void of nothing, only darkness, isolation and a deafening silence. What's more, there is a place in our universe defined by this very notion of nothing. Far off in the distant reaches of the cosmos lies an area so vast and so empty, it has prompted conspiracy theories about hyper-advanced civilisations, it has inspired stories of sci-fi fantasy and it has even taught us a little about the universe itself. It is the universe's desert. A place so vast and empty, all the money on earth converted into pennies wouldn't even fill a fraction of 0.01% of it. What happens when you stare into the abyss and the abyss stares back? The Boötes Void. ## What Actually Is a Cosmic Void? So, one of the first questions we have to ask ourselves in order to understand the Boötes Void is: what is a void? Scientifically speaking, a void in space is a vast region with a very low cosmic mean density. That's a fancy way of saying there's nothing in it, or rather, very little in it. You see, despite being called a void, the Boötes Void is far from totally empty. A void's definition comes from its low cosmic density relative to the rest of space. The Boötes Void, one of the largest in the known universe, is 330 million light years across according to the BBC. It's absolutely enormous. But its size actually gives a good reference point for just how empty it is compared to the rest of space. In an average slice of space that's the same size as the Boötes Void, you would expect to find over 2,000 galaxies in it, meaning that on average there is a galaxy every 165,000 light years or so. In the Boötes Void, there are 60 galaxies. Which means on average, you would only expect to run into another galaxy once every 5.5 million light years. There are great expanses of nothing here on earth—deserts and oceans where you can see absolutely nothing in every direction right to the horizon. Well, the number of earths it would take to fit into the Boötes Void is 1.47×1051. That is a quadrillion, quadrillion, quadrillion earths' worth of space. BBC's *Sky at Night* magazine states that a region the volume of the Milky Way could fit into it billions of times over. All with just 60 galaxies in it. These galaxies are sometimes known as "field galaxies" because they are isolated from the large clusters of galaxies that dominate other regions of the universe. There is something to be said about the idea of being in a galaxy in the middle of an enormous void and the major change in perspective to space that it would cause. If earth was in the middle of the Boötes Void, until we discovered how to map the universe, we would be led to believe that space is almost entirely empty, with our own galaxy being one of the only things in it. If we were in the middle, it's not like we would be able to get our satellites out of the void very easily. We may have discovered and come across a few other galaxies and planets, but certainly not the overwhelming abundance of them in the universe like we come to expect today. As astronomer Greg Aldering put it: "If the Milky Way had been in the centre of the Boötes void, we wouldn't have known there were other galaxies until the 1960s." Voids themselves aren't a new thing, with the first one being discovered in the 19th century. However, nobody at that time could predict the sheer size and scale of universal structures. The Boötes Void is so massive that it takes up around 2% of the diameter of the observable universe. It is referred to as the "Great Void" as well as "The Great Nothing". The name Boötes Void comes from the fact that the void contains the constellation of Boötes, the herdsman who pushes the plough (Ursa Major) around Polaris (the North Star). Poor Boötes, doomed to circle forever in a vast sea of nothing, truly a fate worse than death. The void is also surrounded by the Boötes superclusters, galaxy clusters that exist around the edges of the void, among other galaxies and stars. It's also really far away—a full 700 million light years away. So rest assured, humanity won't be visiting any time soon. With current technology, it would take around 13 trillion years to reach it. If you wanted to cross it, it would be another 6. So sure, it's big and it's empty, but that poses a question. A void is just cosmic nothing; space is already mostly nothing. How did we find out the Boötes Void even existed in the first place? Well, for that, we need to go back to the 1970s and 1980s and to a man who would change science forever: an American scientist by the name of Robert Kirshner. ## The Discovery of the Boötes Void In the 1970s, we were still just dipping our toe into the stars. We had been to the moon, orbited the earth, launched satellites and probes into deep space, but our understanding of space still lacked depth. That is until redshift surveys came along. Redshift surveys are a type of astronomical survey that are used to measure the distribution of interstellar objects by calculating their "redshifts". A redshift occurs when light or other electromagnetic radiation from an object is stretched to longer wavelengths (toward the red part of the light spectrum) as the object moves away from the observer. What is the universe doing at all times? Expanding. So objects are almost always moving further away, stretching that light. We can measure the distance from the earth to the redshifted light of interstellar objects to see how far away they are as well as how quickly they are moving away from us. In effect, this gives us a way to map the observable universe in full 3D. So, in the mid-1970s, redshift surveys were starting to become common in our pledge to understand the stars. By the 1980s is when our hero, Robert Kirshner, came in. In 1981, Robert was working alongside his fellow astronomers at the University of Michigan, as well as at the Harvard-Smithsonian Institute for Astrophysics, in order to calculate the redshift of a large number of galaxies in the observable universe. He would go on to become a professor at the institute later in life. Robert and his colleagues were doing the survey when one day they discovered a huge blank region, bigger than any void that had been seen up to that point, in a portion of the universe around 700 million light years away. In this part of the universe, Kirshner and co expected to see large swathes of redshifted light, and there was in a lot of other areas. But right where we now know the Boötes Void sits, there was simply nothing. Was it a problem with the software? No, the computer was working fine. The group had stumbled across a hitherto undiscovered void—a super void. There were other voids that had been found before this point, as we mentioned redshift surveys go back to the mid-70s, but definitely none this size. It was a real-life marvel, right in our own universe. But also, as I mentioned, it wasn't the only one. Yes, the discovery of the Boötes Void led to major breakthroughs in our understanding of the universe, but it technically isn't the biggest, emptiest thing in the universe. It certainly was at the time it was discovered; however, since then more complete maps have been drawn of the observable universe and it turns out the Boötes Void was not the only super void on the block. The biggest of them all is known as the KBC void—it is a staggering 2 billion light years across, as opposed to the Boötes Void's comparatively puny 330 million. Other voids over a billion light years across do exist, like the Eridanus super void and the Sloan Great Wall void, leaving the great Boötes Void as something of a historical and statistical afterthought by comparison. Voids in space have captivated audiences across a wide range of sci-fi media, from *Star Trek* to Peter F. Hamilton's *Commonwealth Saga*. There is something of a call of the void, a deeply human macabre interest in these areas of space that house so little. Where is the Boötes Void today? Where it's almost always been and will continue to be for a long time yet—near the Boötes constellation, 700 million light years away. Well, as far as we know anyway. The light that reaches our equipment on earth has been travelling for 700 million years. So any images we do have of the void will be primordial. But this can be useful. We can use the Boötes Void and others like it to understand some of the most pressing questions about our universe, and I've got a pretty good one: why is it there? ## Where Do Voids Come From? "That's all well and good," you might say, "but how can we answer the prevailing question of why such a large expanse of space is filled with seemingly nothing at all?" We all know that space is mostly empty—it's why it's called space. However, this significantly sized chunk of it is distinctly emptier. A drop from several thousand galaxies to 60 is over a 95% decrease in matter in the area. That seems like an anomaly, doesn't it? But the sheer size of the thing and the fact we know there are larger ones defies that very notion. Why is this great void of nothing here? Well, our understanding of these voids is like a negative photo. They are defined by what they lack. Scientists have created detailed maps of our universe and when putting the pieces together, it looks like a giant spider's web. The vast majority of galaxies in our universe are found on the threads of this web. They're known as "filaments". Whilst the empty spaces in the middle are known as voids. Have you ever seen a population density map of Australia? That's what one of these individual voids with galaxies around the edge would look like. Except there are thousands in the observable universe, all linked together. So voids are actually relatively common. There's even one near the Milky Way, creatively named the Local Void. They make up 80% of the observable universe and many are millions of light years across. Where galaxy filaments meet at areas of high concentration, it's known as a cluster. We're in one right now. The Milky Way is part of the Virgo cluster, which in turn is part of the Laniakea Supercluster. These filaments and voids are the largest structures in the known universe, and the Boötes Void is simply one of the largest pieces of the puzzle. Ok great, but why does it exist? The BBC's *Science Focus* asserts that in order to understand why we have these voids and filaments, we have to go back to where it all began: the Big Bang. When the universe was first formed, the matter contained within it (which encompasses all the mass in the known universe) was densely packed together because the universe was still small. Scientists' theory is that in this densely packed state, quantum fluctuations within the densely packed matter caused some areas of matter to be slightly more densely packed than others. Denser areas began to gather matter quickly, owed to gravity, pulling in other matter from the less densely packed areas. As the universe continued to expand outwards, these denser areas continued to grow and clump together, eventually forming the galaxies we see today. However, as those less densely packed areas also began to expand, they continued to lose matter through gravity to the more densely packed areas. So over an excruciatingly long period of time, there was less and less matter in those areas, thus creating voids; similarly, there was more and more matter in the clusters, which then created the filaments. This led to the formation of the cosmic web as we know it today. This makes logical sense from a scientific standpoint. Small fluctuations over billions of years would make quite the difference as the universe continued to expand in that time. These individual voids are like bubbles in foam, where smaller bubbles can merge together into larger ones as they expand. Many believe this is how the Boötes Void, alongside the other larger voids in the universe, came to be. These filaments and voids, weaved together like a spider web, have allowed us to map large portions of the observable universe. The clusters of galaxies that make up the filaments give off more energy than the colder, mostly empty voids. This means we can map them when using thermal imaging, which is effectively how we discovered the concept in the first place. Joseph Clampitt, a post-doctoral researcher at the University of Pennsylvania, explained the phenomenon in a novel way using height as a stand-in for density, quoting here: "Voids are vast, shallow troughs in the dark matter [distribution], galaxies sit at the centre of narrow, towering peaks. A typical galaxy has the same height and footprint as the Empire State Building [and] a medium-size void would be a 3-foot-deep hole three times the size of Manhattan." So the Boötes Void is just that—a void. A relic of a time long passed when the universe began, that expanded to become one of the largest things in the observable universe. A large hole in space filled to the brim with nothing at all. Of course, there are a few galaxies, but the Boötes Void is largely a barren wasteland. It conjures up thoughts of what it would be like to exist inside it for even a minute. To look around you and see an endless expanse of darkness in every direction. Not even planets, galaxies or stars. It has a funny allure to it. The void calls for us all. However, there are some that don't believe that the Boötes Void is an afterthought of a tiny fluctuation in energy when the universe began. Instead, they have pioneered a set of theories, some wackier than others, in order to explain why there is a 330 million light year expanse of nothing in the middle of space. Let's explore those, shall we? ## Fringe Theories – The Good, the Bad and the Wacky As our understanding of voids has only really grown since the 1980s—which is relatively recent in scientific terms; we went to the moon in the 60s after all—there are a lot of fringe theories with varying degrees of scientific merit to explain phenomena within the Boötes Void. They vary from the relatively scientifically sound all the way into conspiracy theory nonsense. So, what do people think the Boötes Void is really? We'll start with the obvious one: a black hole. It would make some scientific sense for the Boötes Void to be a black hole. After all, there seems to be so little there relative to other more densely packed parts of the universe that even light can't seem to pass all the way through the void, indicating that the light could be being affected by some massive interstellar object. We know black holes have strong enough gravity to affect and even trap light. So in theory, it could be a reason some voids exist. However, when we start to think about it as an explanation for the Boötes Void in more depth, it just doesn't add up. The largest black hole ever discovered is TON 618. NASA states it's 66 billion times bigger than the sun. Its event horizon is 0.02 light years across. The Boötes Void is 330 million light years across. This is a whole other level of scale. Simply put, if the Boötes Void was a black hole, we would have much, MUCH bigger problems. Not to mention we'd be able to see massive light distortions around that area of space, and one would expect to see the 60 galaxies in the void affected somehow by a massive black hole. Also, the fact that stars collapse into black holes puts this theory to bed. In order for a star to collapse into a black hole, it needs an extremely large amount of mass. Moreover, given what we know about the cosmic web, where most of the universe's mass is concentrated along dense filaments, it's unlikely that anything with significant mass like a black hole could be present in the Boötes Void. Now I'll grant you, that doesn't make it impossible for there to be an enormous black hole in the Boötes Void that scientists somehow simply haven't found for some reason. After all, there are stars in it, and a large enough one could cause a black hole if it collapsed. However, what we know of the formation of these voids simply makes this hypothesis less likely. In a similar capacity, some have theorised that the Boötes Void is a dark nebula, which are molecular clouds so dense that they obscure visible light, masking the light behind them. This too makes some sense, but again there is a size mismatch here. The biggest dark nebula we know of, the Coalsack Nebula, is only around 30 light years across versus the 330 million for the Boötes Void. We also run into the same issues with the 60 galaxies in the void—a dark nebula would obscure them from view. Finally, given that dark nebulae are often found in galaxy structures, and given the relative lack of galaxies in the void, it's highly unlikely one could grow large enough to explain the Boötes Void. Alright, we've done some proper ones, now let's do the silly stuff. Some have claimed that the Boötes Void actually doesn't exist. That's right—a 330 million light year void in space actually isn't real. What is there, you may ask? Dyson spheres. A Dyson Sphere is an idea crafted by physicist and mathematician Freeman Dyson in 1960. The idea goes that it could be possible for advanced civilisations to build a superstructure around a star in order to harness as much of its energy as possible. These superstructures are known as Dyson Spheres. I'm sure you can see where this is going. It's asserted that the Boötes Void is not a void at all. It is in fact a hyper-advanced civilisation thousands of years ahead of humans in its development. Not only has this civilisation pioneered interstellar travel and Dyson Sphere building, but mastered it. They mastered it to such an extent that there is now a 330 million light year dark patch in space, as most of the stars have been covered up. Now we don't need to dive into a complex analysis on this one; we simply have to play a short game of Occam's Razor. What's more likely: a civilisation advanced enough to cover 330 million light years' worth of stars in Dyson Spheres (amounting to billions of stars), or there's a bit of space that's emptier than the rest of space, which is already 99% empty? I rest my case, your honour. One of the more minor theories about large voids in general is that they have some kind of special interactions with dark energy and antimatter. Another asserts that large voids contain remnants of "bubble universes". However, rather than just for silly goofs or as wild speculation, there is one theory about large voids that may tell us a lot about the fate of the universe itself. As we understand it, those voids were formed when all of the mass in the universe was packed tightly together before the Big Bang. When the universe expanded, the minor quantum fluctuations eventually became a web of mass that litters the cosmos, and the spaces between the filaments were the voids. Some scientists theorise that the universe could keep expanding indefinitely, while others think it will reach a certain size and then retreat back in. However, for those who think the universe will continue to expand, it gives us a potential window into the future of the universe itself. The large voids we see in the universe tell us that this is the result of a massive expansion over a very long period of time. If the universe is ever-expanding, however, and will continue to do so in perpetuity, then the distances between matter will continue to grow indefinitely too. After billions or maybe even trillions of years' time, stars will burn out, galaxies will become isolated, and the universe may reach a state of near-total emptiness, not unlike what we see in the Boötes Void today. In that sense, Boötes could be a window into the long-term fate of the cosmos. ## What's So Special About the Boötes Void Anyway? Ok, so we've talked about the Boötes Void. We've established it's likely a semi-common phenomenon that happens throughout the universe. We've learned it's not even the biggest void that we know of in space. So why is it special? Why focus on this specific void if it's no different from the others except it's a little bigger than most of them? Well, because in a funny way, the Boötes Void is a gift. You're right—in the grand scale of the universe, the Boötes Void is like any other. In the cosmic expanse it may not matter, but to us, it's special. The Boötes Void was not just some void we discovered; it is one of the first voids that was ever discovered and certainly the largest at the time. So it's partially significant for that reason, but that's not all. Kirshner's work has since been built upon. Not only can voids potentially indicate a direction of travel for our universe in the future, but they also give us an indication of our past. In studying these structures, astrophysicists today can gaze through a window into what the universe looked like in its earliest moments. BBC's *Science Focus* states that advancements in both telescope and imaging technology have made it possible to create more detailed versions of Kirshner's maps, such as the Dark Energy Survey which has mapped out a quarter of the southern sky and examined around 300 million galaxies. Meanwhile, supercomputers can now create detailed simulations of the universe's expansion. By comparing these maps with the simulations, astronomers can begin to understand how our universe came to look the way it does today. In no short order, the Boötes Void altered the way we think about the universe. It has helped make up one small nugget of our understanding of our own universe, which by proxy helps you and I understand our place in it all. So I'm grateful to that 330 million light year void, because without it, we may not understand just how incredibly fortunate we are to be here today. Simply put, the Boötes Void may be a huge expanse of nothing, but it's given us a piece of our everything. ## Key Takeaways - The Boötes Void is a vast, nearly empty region of space, 330 million light-years across. - It contains only 60 galaxies, far fewer than expected for its size. - The void was discovered in the 1980s using redshift surveys, which map the universe in 3D. - Cosmic voids like Boötes are formed from quantum fluctuations during the Big Bang. - The Boötes Void has significantly advanced our understanding of the universe's structure and expansion. ## Frequently Asked Questions ### What is the Boötes Void? The Boötes Void is a vast region in space with a very low cosmic mean density, making it one of the largest and emptiest known voids in the universe. It is approximately 330 million light years across and contains only about 60 galaxies. ### How was the Boötes Void discovered? The Boötes Void was discovered in the 1980s by Robert Kirshner and his colleagues using redshift surveys. They noticed a large blank region in the universe where they expected to see redshifted light from galaxies. ### What are cosmic voids? Cosmic voids are vast regions in space with a very low cosmic mean density. They are defined by their low density relative to the rest of space and are part of the cosmic web structure of the universe. ### How many galaxies are in the Boötes Void? The Boötes Void contains approximately 60 galaxies, which is significantly fewer than the expected 2,000 galaxies in a similarly sized region of space. ### What is the significance of the Boötes Void? The Boötes Void is significant because it was one of the first large voids discovered and has contributed to our understanding of the universe's structure and expansion. It provides insights into the early moments of the universe and the potential future of cosmic expansion. ### What are some fringe theories about the Boötes Void? Some fringe theories suggest that the Boötes Void could be a black hole, a dark nebula, or even a region filled with Dyson spheres built by an advanced civilization. However, these theories are generally considered less likely based on current scientific understanding. ### How does the Boötes Void relate to the cosmic web? The Boötes Void is part of the cosmic web structure, where most of the universe's mass is concentrated along dense filaments, and the empty spaces between these filaments are voids. The Boötes Void is one of the largest voids in this web. ### What is the distance to the Boötes Void? The Boötes Void is located approximately 700 million light years away from Earth, making it one of the most distant and largest known voids in the observable universe. ### What are redshift surveys and how are they used to study the universe? Redshift surveys are astronomical surveys that measure the distribution of interstellar objects by calculating their redshifts. This helps in mapping the observable universe in 3D by determining the distance and velocity of objects moving away from Earth. ### What is the potential future of the universe based on the study of voids? The study of large voids like the Boötes Void suggests that the universe may continue to expand indefinitely, leading to a state of near-total emptiness over billions or trillions of years. This provides a window into the long-term fate of the cosmos. ## Sources - [Original Places video: The Boötes Void: The Biggest, Emptiest Thing in the Universe](https://www.youtube.com/watch?v=Z2xGOvUIX68) - [Hero image source](https://images.rawpixel.com/editor_1024/czNmcy1wcml2YXRlL3Jhd3BpeGVsX2ltYWdlcy93ZWJzaXRlX2NvbnRlbnQvbHIvdXB3azYxNjkzODU2LXdpa2ltZWRpYS1pbWFnZS1rb3dsbDVsMS5qcGc.jpg) by openverse, cc0. ## Related Coverage