The Boötes void, named after the constellation where it can be found, is the largest known region of empty space in the observable universe. Because it is so large, it is sometimes referred to as a supervoid. The void is roughly spherical and has a diameter of approximately 75 megaparsecs, or 250 million light-years, which is about 2% the diameter of the entire observable universe(!)

For comparison, our Milky Way Galaxy has a diameter of about 100,000 light-years, and the largest known galaxy is about 250,000 light years across, a thousandth the width of the void. Within this vast emptiness, only about 53 luminous galaxies have been detected, which extend in a rough tube-shape through the middle of the void. Other galaxies surely exist within the void, including structures of dark matter, but these galaxies are smaller and less massive than the 53 primaries. These 53 galaxies have an average brightness about 25% more intense than the universal average, a phenomenon that needs explaining.

Greg Aldering, an astronomer who now works at Lawrence Berkeley Laboratory, once said, “If the Milky Way had been in the center of the Boötes void, we wouldn’t have known there were other galaxies until the 1960s.” Imagine that kind of a discovery!

The void was discovered in 1981 by Robert Kirshner, Augustus Oemler Jr, Paul Schechter and Stephen Shectman in a survey of galactic redshifts. Their results were published in the paper, “A million cubic megaparsec void in Bootes” in Astrophysics Journal 248. Further studies throughout the early 80s confirmed the existence of the void, which was one of the first large voids to be detected, and is hence the most famous.

Telescopes pointing in the direction of Boötes show a sky with plenty of galaxies. What the studies showed was that all of these galaxies are either close to us or far away, with a gigantic gap in between. This gap is the Boötes void, whose center lies about 700 million light-years distant.

The Boötes void is probably the most perfect vacuum in the universe. Its density is somewhat less than that of the universe’s average, which is about one atom per cubic meter. The void’s density is certainly lower than that of typical intergalactic space, which is already extremely sparse. Like the rest of space, the most plentiful form of conventional matter to be found within the void is ionized hydrogen.

Think about it: the density of lead is 11.34 g/cm³. The density of the Boötes void is about 1.674× 10 ^ − 29 g / cm³, approximately a million million million million million times more diffuse than that. You might have heard that a neutrino of the type emitted by the Sun can be blocked by a barrier of lead about two light years across. To block such a neutrino with a barrier equal in density to the Boötes void would require the barrier to be quadrillions of times wider than the observable universe.

This extremely low density means that when a pattern of neutrinos enters in one side of the void, it looks exactly the same upon exit. Same goes for photons. Particles of matter, having much more mass than both photons and neutrinos, would of course get pulled towards the walls of the void. Because of this state-preserving property, the Boötes void may one day be seen as the ultimate time capsule - fire off a pattern of photons, only for the pattern to be rediscovered hundreds of millions of years later when it reaches the other side.

This void is so big and empty, its discovery worried cosmologists. The prevailing theory of galaxy formation suggested a more uniform distribution of matter throughout the universe, in the spirit of the cosmic microwave background radiation, which is the roughly uniform “echo” of the Big Bang. The Boötes void fell outside the range of void sizes predicted by this theory of galaxy formation. Cosmologists are still trying to create theories that better explain it.

The size of the Boötes void is too large for it to have formed as a result of natural galactic rearrangement from gravitational factors. It had to have been “set up” to be a void from the beginning of the universe. Perhaps the supervoid began as a particularly large quantum ripple on the surface of whatever exploded to become our cosmos. The Boötes void may be in that class of phenomena that requires a quantum theory of gravity to explain. Though today it is extremely large, its origins are grounded in an object many times smaller than the atomic nucleus.

There are two main theories of galaxy formation - “top-down” theories, where large structures form first and then fragment into smaller ones, or “bottom-up” theories, where large structures are formed by the coalescing of little bits. The existence of features on the scale of the Boötes supervoid is evidence in favor of the former. If the galactic formation mechanism were primarily bottom-up, the universe would be more like pudding - smooth and evenly distributed. But instead it looks more like a froth of soap bubbles, with galactic filaments separating gaping voids.

The current prevailing theory of galaxy formation is the Lambda CDM (cold dark matter) model, which is a bottom-up model.

The Boötes void is the utter absence of structure on the scale of galactic superclusters. To review, extragalactic astronomy recognizes several levels of organization of matter in the universe, beginning with galaxies, with a characteristic length of 20 kpc, which can be found within a group or cluster that measures 50 kpc to 5 Mpc across, which are further organized into superclusters with diameters topping 50 Mpc. The Boötes void is 75 Mpc across. Many other voids exist, but none this large that we have yet seen.

It is known that about 98% of the volume of the universe is consumed by intergalactic voids. The universe is made up of superclusters forming thin “walls” around these huge voids, perhaps reminiscient of the way organisms consist of cells whose main density lies in walls enclosing cytoplasm. (But in contrast to the way that atoms’ primary concentration of density is located in the nucleus.)

Of course, certain things can be found within the void, mostly in the form of energy. The extragalactic background light (EBL) and cosmic microwave background radiation (CMB) fill up the void. Interestingly, there is no gap in the cosmic background radiation in the region of the void. The void almost certainly contains a lot of dark matter and energy, perhaps even at densities no different than those found within superclusters. This means that dark galaxies and dark energy stars can probably be found. And, of course, the void is filled with endless quantities of virtual particles, which are created and annihilated constantly on the smallest timescales.

According to the Wikipedia article on the topic, the Boötes Void was mentioned in a novel by Martin Amis, “Night Train”, which centers around the mysterious suicide of a beautiful and successful astrophysicist Jennifer Rockwell. The immense size of the void leads her to conclude that there is no meaning to life, so she kills herself.

What does contemplating the size of the void lead us to conclude?

First is the logarithmic scale of human awe, which seems to be built into our brains, and is arguably a common feature in any brain built by evolution and natural selection. The Boötes Void is not that many times more impressive, subjectively, than a cave so large you can fly a helicopter around inside. Both are big, awe-inspiring, and amazing. Despite this, most people would be more impressed by the big cave, because it’s something that we can more easily imagine ourselves interacting with, something that has features similar to those we run across on a daily basis - rocks, ceilings, walls, passageways, etc. But even if you added together every cave on every planet in the universe, you’d still fall many orders of magnitude shy of the volume of that void of all voids.

It might be possible to one day build a mind that experiences a linear increase in awe with every linear increase in size, for any given void or chasm. To such a mind, a 20 cubic meter hole would be twice as impressive as a 10 cubic meter hole. I can only assume that contemplating the Boötes Void would cause this mind to self-destruct.

Secondly, the void brings to mind that everything is relative. This point is similar to the first. For example, consider a reversible agent sent into the void at relativistic speeds. A reversible agent recovers all its lost energy perfectly, so it can observe and think forever without consuming a joule. As the light of nearby galaxies slowly faded into almost complete darkness, this agent would get pretty damn bored. Most likely it would try communicating with people in the galaxies on the edge on the void, but the response time would deteriorate quickly. If the agent is lucky, it will have a very long-lived and extremely patient pen pal with which to exchange messages and play games. The agent might choose to deliberately slow down its rate of thought so that time passes more quickly. If your mind is operating slowly enough, a trip across the void might only seem like a mere decade or even a week.

Equivalently, an observer accelerated close to the speed of light experiences time more slowly. If you were accelerated to a velocity arbitrarily close to that of light speed, the trip could seem arbitrarily short. According to the Wikipedia article on the topic, a continuous acceleration of 1 g would be sufficient to allow a person to cross the entire observable universe in a subjective duration less than a normal lifetime. A similar level of acceleration would allow a traveller to cross the void quickly, but some form of braking would be advisable. At such high speeds, a typical galaxy would look like a wall of high-energy cosmic rays.

Perhaps the Boötes void will serve as a proving ground for the high-speed dragsters of the future, moving at 99.99999% the speed of light. Its extremely low density would certainly be appealing to anyone who wants to set speed records without that pesky intergalactic dust getting in the way.

A third line of speculation about the void would be the presence or absence of life within its confines. Of course, like the rest of the universe, the void is almost certainly empty, for anthropic reasons. (The probability of any given species having a neighbor is equal to the number of universes with at least two intelligent species divided by the number of universes with at least one intelligent species, which is a huge ratio.) However, if life evolved here, what would it be like? It would probably be made out of building blocks of lone hydrogen atoms floating in the void, exchanging data with photons and other quanta (gravitons?) Over an extremely long length of time, patterns of these atoms and quanta could become self-propagating, leading to a cycle of variation and evolution. However, if this were possible, it would probably happen in plain old interstellar or intergalactic space before it happened in this blank darkness. So much for that idea.

For more information, see a survey of the Boötes void. The symbolic rival of the void is arguably the Great Attractor, a cosmic aggregate of tremendous mass. For a gosh-wow narrative of the density of intergalactic space, see this article.