Inside the Boötes Void

 NEXT HORIZON  |  THE DARKEST MYSTERIES OF THE UNIVERSE  #4

Boötes Void — The Terrifying Emptiness in the Universe

A region hundreds of millions of light-years across appears almost blank on maps of the cosmic web. The Boötes Void is not truly empty, but that may be the more interesting story: how did gravity help create such a vast underdensity, what survives inside it, and what can cosmic emptiness tell us about the Universe?

Boötes Void is one of the most famous cosmic voids — a vast underdense region embedded in the large-scale structure of the Universe.

The empty place that changed how we see the Universe

Imagine the Milky Way sitting near the middle of an enormous region with very few bright galaxies nearby. The night sky would still glitter with stars - almost all the stars we see with the naked eye belong to our own galaxy. But beyond the Milky Way, the view would be strangely sparse. The neighbouring galaxies and clusters that helped astronomers understand our place in the cosmos would be harder to find.

That thought experiment gets close to what makes the Boötes Void so unsettling. On maps of the large-scale Universe, it looks like a missing patch: a huge region in the direction of the constellation Boötes where galaxies are far scarcer than expected. Popular accounts sometimes call it the “Great Nothing.” The nickname is memorable, but it is also misleading. Boötes is not a hole in space, a black bubble or a place where physics stops working. It is an extreme example of something common in the Universe: a cosmic void.

The real shock is scale. A Universe that looks nearly uniform when viewed from far enough away can still arrange matter into walls and filaments separated by cavities so large that light needs hundreds of millions of years to cross them.

First: what is a cosmic void?

Galaxies are not scattered through space at random. On scales of tens to hundreds of millions of light-years, matter forms a vast three-dimensional network known as the cosmic web. Galaxy clusters gather at dense intersections. Long filaments connect them. Broad sheets form walls. Between these structures lie the underdense regions we call cosmic voids.

A void is defined by comparison. It contains less matter - and usually far fewer bright galaxies - than an average region of the same volume. But less is not none. Voids still contain dark matter, diffuse gas and galaxies, including faint systems that are easy to miss. Their boundaries are also fuzzy. Change the survey, the detection limit or the mathematical void-finding method, and the exact edge can shift.

That is why a single, perfectly precise diameter or galaxy count can give a false sense of certainty. A cosmic void has no solid wall. Its size depends partly on how we choose to map it.

How astronomers discovered something that was not there

The Boötes Void emerged from a redshift survey led by Robert Kirshner, Augustus Oemler Jr., Paul Schechter and Stephen Shectman. Redshift tells astronomers how fast a distant galaxy is receding as the Universe expands. With enough measurements, a flat map of points on the sky becomes a rough three-dimensional map of where galaxies actually lie.

In their 1981 paper, the team reported redshifts for 133 galaxies in three northern fields. If galaxies had been distributed fairly uniformly, many should have appeared around a recession velocity of roughly 15,000 kilometres per second. Instead, across a 6,000 km/s-wide interval centred near that value, they found only one. The researchers suggested they had encountered a region of roughly one million cubic megaparsecs that was almost devoid of ordinary bright galaxies.

That was a serious surprise. Astronomers were still piecing together the geometry of large-scale structure, and a gap this large raised a basic question: was Boötes an odd exception, or was the Universe itself built with enormous empty regions?

A larger follow-up survey published in 1987 made the case much stronger. The team measured redshifts for 239 galaxies selected from 283 small fields and described a roughly spherical underdensity with a radius of about 62 megaparsecs under the assumptions used at the time. Boötes was no statistical accident.

Astronomers first identified the Boötes Void not as an object, but as a striking absence in redshift surveys.

A note about the famous size Popular descriptions often put the Boötes Void at roughly 250-330 million light-years across, while the 1987 survey quoted a radius of about 62 Mpc - roughly 200 million light-years. These figures are not directly interchangeable. Historical measurements used different cosmological assumptions, and a void's boundary shifts with the galaxy sample and the method used to define it. The safest description is that Boötes is an enormous, roughly hundred-megaparsec-scale underdensity rather than an object with one exact edge.

 

So how empty is the Boötes Void?

This is where the popular version of the story becomes too tidy. One often-repeated claim says that only about 60 galaxies have been found inside the Boötes Void, compared with thousands that might occupy a similar volume elsewhere. The comparison conveys the right idea - an extreme shortage of luminous galaxies - but the number 60 is not a modern, complete census of everything inside a sharply defined sphere.

Early surveys could see only galaxies bright enough for the instruments and catalogues of their era. In one 1987 emission-line search, just eight galaxies were known in the void. Later optical, infrared and radio observations kept adding systems that the first magnitude-limited surveys were never designed to find. Every new wavelength changed the census - and, with it, the apparent depth of the void.

The durable conclusion is simpler: Boötes contains far fewer ordinary luminous galaxies than an average region of comparable size. Even the discovery-era measurements were consistent with a galaxy density no more than roughly one-tenth of the surrounding background. The void is dramatically underdense. It is not sterile.

“Empty” depends on what you use to look

A deep IRAS redshift survey published in 1990 found 12 infrared-selected galaxies inside the region as then defined, seven of them newly identified. For that infrared-bright population, the inferred density was not one-tenth of normal but somewhere between roughly one-sixth and one-third of the cosmic average. The void was still real; the lesson was that different galaxy populations trace it differently.

Radio observations made the point even more sharply. A 1996 Very Large Array survey targeted 24 fields around known Boötes galaxies and covered about 1,100 cubic megaparsecs - only around one percent of the void volume used in the study. Sixteen targets were detected in neutral hydrogen, and the survey found 18 previously uncatalogued companion galaxies directly through their H I emission. Because the fields were centred on known systems, this was not an unbiased census of the whole void. But it proved that gas-rich galaxies could hide from optical catalogues while remaining visible at 21 centimetres.

This distinction matters far beyond Boötes. Astronomers often map the cosmic web using galaxies as tracers, but galaxies are not the matter field itself. A region can be extraordinarily poor in bright galaxies while still containing diffuse gas, dark matter and many low-mass haloes. A “galaxy void” and a perfect vacuum are very different things.

A region that looks almost empty in visible light can still contain gas, dark matter halos and faint galaxies invisible to early surveys.

What actually exists inside the “Great Nothing”?

Dark matter is still there. Its density is lower than average, but it does not vanish inside a void. Over cosmic time, gravity draws matter toward denser surroundings - toward filaments, sheets and clusters - leaving the interior progressively more rarefied. Gas and galaxies broadly follow the same large-scale flow.

Galaxies survive there too. Void galaxies tend to be relatively isolated and are often low in mass, gas-rich, blue and actively forming stars. That makes them useful natural experiments. What happens to a galaxy when it spends billions of years with fewer neighbours, fewer mergers and less environmental disturbance?

The Boötes Void now offers a particularly good example. In 2026, astronomers published a detailed cold-gas study of the disk galaxy CG 910 using CARMA carbon-monoxide observations and 21-cm data from the Green Bank Telescope. They measured a molecular-gas mass of about 1.2 × 10¹⁰ solar masses in a disk roughly 7 kiloparsecs across, plus about 3.1 × 10⁹ solar masses of atomic hydrogen. For comparison, the galaxy’s stellar mass is about 2.15 × 10¹⁰ solar masses.

The CO velocity field is remarkably regular, with a flat rotation speed near 256 km/s and no clear signature of a recent interaction or fresh gas-accretion event. Yet CG 910 is not rapidly burning through that reservoir: its relatively modest star-formation rate implies a long gas-depletion time. In other words, a galaxy deep in a void can hold a large cold-gas supply and still evolve slowly and quietly.

So the interior is not "nothing." It is a low-density environment where galaxies can evolve under unusually isolated conditions.

There is a smaller cosmic web inside the void

The most surprising Boötes result may be that “isolated” does not necessarily mean alone. In the 1996 H I survey, the number of companions within about 1 Mpc of the targeted void galaxies was similar, within a factor of two, to the control sample in regions near the mean cosmic density - even though the large-scale environments differed in density by roughly a factor of six. On scales of a few megaparsecs, the researchers inferred gravitationally bound pairs, loose groups and even one system compact enough to resemble a Hickson compact group.

Follow-up spectroscopy reinforced that picture. A 2002 study of 26 known Boötes galaxies classified 14 as H II galaxies, identified two extreme starbursts and found two previously unrecognised closely interacting pairs. At least five active galactic nuclei were known in the sample by then. The sample was strongly selected toward emission-line and infrared-bright galaxies, so those fractions should not be treated as a census of all void galaxies. But the result destroys the cartoon of a void as a place where nothing ever meets anything else.

Modern simulations explain why. Voids are hierarchical structures. Their interiors can contain low-mass haloes, delicate filaments and small sheets - a faint sub-web nested inside the larger cosmic web. Matter is sparse, not perfectly smooth. A galaxy can live in a globally empty environment while still having a close neighbour, a gas filament or a small local group.

Even inside a giant void, gravity can preserve a faint internal web of filaments, pairs and small galaxy systems.

Why does a void this large exist?

The explanation begins in the early Universe. Matter after the Big Bang was distributed almost uniformly, but not perfectly. Tiny density fluctuations were present everywhere. Some regions began slightly denser than average; others slightly thinner.

Gravity magnified those small differences. Overdense regions pulled in more matter and eventually produced galaxies, clusters and filaments. Underdense regions fell behind. Matter streamed toward denser structures, while the low-density zones became emptier relative to the cosmic average and expanded faster than their surroundings.

No explosion had to clear Boötes out. Nothing carved a spherical cavity through space. A void is the long-term outcome of gravitational evolution acting on a region that started with a little less matter.

Large voids can also grow hierarchically. Smaller underdense regions may effectively merge as the thin structures between them drain away. Cosmologists call this part of "void-in-void" evolution. It provides a natural route to enormous low-density regions without invoking any exotic event.

A void is not a smooth bubble

The word “void” encourages the wrong mental image: an empty sphere with a clean edge. Real voids have radial structure. Their deepest regions are strongly underdense, galaxy density generally rises toward the outskirts, and some develop an overdense ridge or “wall” where evacuated matter accumulates. Recent simulation-and-SDSS comparisons find that these stacked density profiles become deeper and their surrounding walls more pronounced toward the present day.

There are also two broad evolutionary fates. A large underdensity embedded in an underdense environment can keep expanding and absorb smaller neighbouring voids - the classic void-in-void process. A smaller void embedded inside a sufficiently overdense region can instead have its expansion slowed or even be squeezed as the surrounding structure collapses, a process known as void-in-cloud. Boötes is famous for its scale, but the same hierarchy that shapes small voids also helps explain why a giant region need not have a simple spherical anatomy.

Where are all the dwarf galaxies?

This leads to one of the most interesting problems in void physics. Cold-dark-matter simulations predict that even very empty regions should contain many low-mass dark-matter haloes. Yet luminous dwarf galaxies are much rarer than a naive one-galaxy-per-halo picture would suggest. This mismatch has often been called the “void phenomenon.”

A leading explanation is that many small haloes are simply bad at making visible galaxies. After cosmic reionization, ultraviolet radiation heated the intergalactic gas and made it harder for shallow gravitational wells to capture and retain baryons. Hydrodynamic simulations find strong suppression of gas accretion in haloes with circular velocities of only a few tens of kilometres per second, with the exact threshold depending on the thermal and feedback model. Supernova feedback and inefficient cooling can push in the same direction.

That means a void can contain far more gravitational structure than its visible galaxy count suggests. The missing dwarfs are therefore not a side issue: they connect the Boötes story to one of the central questions of galaxy formation - why some dark-matter haloes light up with stars while others remain almost completely dark.

Is the Boötes Void a threat?

No. Its size is intimidating; its gravity is not.

A black hole is dangerous because a great deal of mass is concentrated into a tiny region. A cosmic void is almost the opposite: it contains less matter than average across an enormous volume. It does not suck galaxies inward. On the largest scales, matter tends to flow away from the underdense interior and toward the denser structures around it.

This motion is sometimes described as voids "repelling" matter. That is convenient shorthand, not a new force of nature. Ordinary gravity, acting in an expanding Universe, is enough to explain the effect.

If the Milky Way could somehow be moved into a large void without being disturbed, the Solar System would not be torn apart. Our local galaxy would remain intact. What would change most dramatically is the Universe beyond it.

What would the sky look like from the middle?

The first surprise is that the sky would not look empty. Nearly every naked-eye star belongs to the Milky Way, so our constellations would still be there if the galaxy itself were unchanged. The Milky Way's bright band would still cross the sky. Planets, nebulae and star clusters would look familiar.

The difference would begin beyond our galaxy. Large neighbours would be rarer and farther away, and telescopes would have to look deeper before revealing a rich population of external galaxies. In such an environment, the historical discovery that the Milky Way is only one galaxy among many might have taken much longer.

The cosmic microwave background would still surround us, because it does not come from nearby galaxies. Distant structure would still exist beyond the void. We would not be cut off from the Universe - only unusually far from its nearest major concentrations of galaxies.

The bigger discovery: the Universe is mostly web and void

In 1981, the Boötes gap looked extraordinary. The 1987 confirmation was so striking that the authors noted its depth was hard to reconcile with several then-popular models for how cosmic structure grew. The important historical twist is that the theory changed. Larger redshift surveys, improved simulations and the rise of the modern Lambda-CDM framework showed that large underdensities are not pathological holes punched into an otherwise smooth Universe. They are a natural part of hierarchical structure formation.

Voids occupy a large share of cosmic volume while containing only a minority of the matter. That makes them essential, not incidental. What once looked like unused space turned out to be one of the defining components of large-scale structure.

That broader picture also changed Boötes' status. It remains famous because of its scale and its place in the history of observational cosmology, but modern surveys have found many enormous underdensities and supervoid-like structures. Describing Boötes as simply "the largest void in the Universe" is therefore risky. The answer depends on the survey, the algorithm and even on whether neighbouring underdensities are treated as one structure or several.

Why cosmologists study emptiness

Galaxy clusters are spectacular, but scientifically they are messy. Galaxies collide and merge. Hot gas fills the environment. Feedback from stars and black holes reshapes visible matter. Dense regions carry the scars of many overlapping processes.

Voids offer a different kind of laboratory. Because their environments are less crowded, the growth of structure can be easier to disentangle. Their sizes, shapes, galaxy motions and lensing signals preserve information about how cosmic structure evolves. In large statistical samples, voids can help test the standard Lambda-CDM model and probe quantities connected to matter density, dark energy, neutrino mass and possible alternatives to general relativity.

A 2025 study offers a good example. Using roughly ten million luminous red galaxies from the DESI Legacy Survey, researchers measured the weak gravitational-lensing imprint of cosmic voids in the Planck 2018 lensing map. The combined signal was detected at high significance and agreed with Lambda-CDM simulations. Regions defined by a lack of galaxies had become precision cosmology tools.

Another study published in 2026 compared voids in the Millennium Simulation with Sloan Digital Sky Survey data to track how these structures evolve. It found the expected broad trend: void interiors become emptier toward the present, galaxies move outward and surrounding walls grow more pronounced. But it also reported galaxies near the centres of observed voids that were not reproduced in the simulated sample. Whether that reflects galaxy-formation physics, sample selection or modelling details, it shows why the emptiest environments remain useful tests.

How do you use emptiness as a measuring instrument?

One method is weak gravitational lensing. A void contains less mass than average, so background light is deflected in a subtly different way as it passes through and around the underdensity. Stack many voids together and that tiny signal becomes measurable, giving astronomers a way to estimate the missing mass directly rather than inferring everything from the absence of bright galaxies.

A second method uses galaxy motions. Matter tends to flow outward from void interiors toward denser walls and filaments. Those coherent velocities distort the apparent shapes of voids in redshift space. Modelling the distortion provides information about how quickly cosmic structure is growing - and therefore about gravity and the expansion history.

A third approach is geometrical. When many voids are stacked, their average shape should have no preferred direction in real space. If astronomers convert redshifts and angles into distances using the wrong cosmology, the stacked voids appear systematically stretched or squashed along the line of sight. This is the Alcock-Paczynski test, and voids provide an unusually clean way to apply it.

The 2026 review literature now describes this as an era of precision cosmology with voids. Their abundance, density profiles, lensing signal and velocity field can be combined to constrain dark energy, neutrino mass and possible departures from general relativity. None of that requires the Boötes Void itself to be unique. The power comes from treating famous systems such as Boötes as members of a statistically measured population.

Cosmic voids are now used as tools for testing dark energy, gravity and the growth of structure in the Universe.

What is still mysterious about the Boötes Void?

The basic existence of Boötes is no longer a mystery that demands exotic physics. Standard structure formation naturally produces voids. The interesting questions have shifted from "How can this exist?" to "What, exactly, is living there - and does it behave the way our models predict?"

How many faint galaxies are still hidden there?

Every census has a detection limit. Dwarf galaxies with little starlight are far harder to find across hundreds of millions of light-years than bright spirals. Radio observations can uncover gas-rich systems that optical surveys miss. As instruments improve, the famous "about 60 galaxies" line becomes less a final answer than a snapshot of what earlier surveys could see.

How different are void galaxies really?

Environmental trends are real, but they are subtler than the old “void galaxies are young blue dwarfs” stereotype. A 2025 MaNGA-based study found that, after matching samples, void galaxies tended to host younger and less metal-rich stellar populations than galaxies outside voids. At the same time, it found no significant overall difference in gas mass between the environments. Stellar mass, morphology, satellite status and local companions can matter as much as the large-scale label “void.”

That complexity is important for Boötes. Its interacting pairs and compact groups show that local environment can survive inside a globally underdense region, while CG 910 shows the opposite extreme: a massive, gas-rich disk with a remarkably regular velocity field. There is no single evolutionary template for a void galaxy.

Can a galaxy shut down even when almost nothing is nearby?

In 2025, astronomers reported isolated quenched dwarf galaxies in some of the least dense parts of the cosmic web, including void environments. The objects had no neighbouring galaxy within about 1 Mpc in projected distance and had formed essentially no new stars for roughly two billion years. That is difficult to explain with environmental stripping alone. Internal feedback, black holes, early formation history or unusual gas conditions may sometimes shut a dwarf galaxy down without help from a cluster.

Do simulations reproduce the emptiest regions correctly?

Lambda-CDM simulations readily produce a cosmic web filled with voids, including internal substructure and coherent outward flows. But modelling the galaxies inside the thinnest environments is harder than modelling the dark matter. Feedback, reionization, gas cooling, dwarf-galaxy formation and survey selection all affect the comparison. A 2026 simulation-SDSS study reproduced many broad trends yet found a non-zero population of galaxies near observed void centres that was absent from its simulated sample. That is not evidence that Lambda-CDM has failed; it is a precise clue about where galaxy-formation prescriptions and observational selection can be tested most severely.

What the next decade may reveal

The next advance will come from combining enormous galaxy maps with measurements that do not depend on visible starlight alone. DESI is building a vast three-dimensional spectroscopic map. Euclid is measuring galaxy positions and gravitational lensing across a large fraction of the extragalactic sky. The Vera C. Rubin Observatory will repeatedly image faint galaxies on an unprecedented scale, while radio facilities and Square Kilometre Array pathfinders can trace neutral hydrogen in gas-rich systems that optical surveys may miss.

The key change is statistical power. Instead of treating a handful of famous voids as curiosities, cosmologists can compare huge populations across cosmic time: large and small, isolated and embedded, young and evolved. They can measure how voids lens background light, how fast galaxies move around them and whether the populations inside match simulations.

For Boötes specifically, the most valuable next step may be less glamorous than finding a new “largest void.” It is a deeper, less biased census: faint optical galaxies, H I-rich systems, molecular gas, stellar populations and weak-lensing mass maps measured together. That would tell us whether the galaxies already known are representative residents of the underdensity or simply the bright, gas-rich exceptions easiest to detect.

Boötes will remain one of the best-known examples, but increasingly it will be understood as part of a much larger population - a historically important doorway into a Universe shaped as much by low-density regions as by the galaxies that first drew our attention.

The frightening part is not the emptiness

The Boötes Void is often framed as cosmic horror: a gigantic region of darkness with almost nothing inside. The image works because the scale is difficult to process. Light can cross the Milky Way in roughly a hundred thousand years; crossing a structure on the scale of Boötes takes hundreds of millions.

The science, however, is more interesting than the horror-story version.

Boötes is not an absence of the Universe. It is one of the structures the Universe produced. The same gravitational evolution that helped build dense galaxy clusters also drained matter from underdense regions. The galaxies that remain there still contain stars, gas, black holes and ongoing evolution. And the very lack of matter that once made voids look unimportant now makes them sensitive probes of cosmology.

That is the real lesson of Boötes: even cosmic "nothing" has a history, an environment and measurable physics.

On the largest scales, emptiness is not simply what is left over after the Universe is built. Emptiness is part of what the Universe builds.

Read also in The Darkest Mysteries of the Universe: False Vacuum Decay - Could the Universe End Without Warning?

FAQ

Is the Boötes Void completely empty?

No. It is an extremely underdense region, not a perfect vacuum. It still contains dark matter, diffuse gas, low-mass haloes and galaxies. Radio surveys have even found previously uncatalogued gas-rich companions around known Boötes systems.

How big is the Boötes Void?

There is no single exact diameter because a void has no hard boundary. Popular descriptions often quote roughly 250-330 million light-years across, while the 1987 survey described a roughly spherical underdensity with a radius of about 62 Mpc under the assumptions used at the time.

How far is the Boötes Void from Earth?

Its centre is at a redshift corresponding to a distance of several hundred million light-years; popular estimates are around 700 million light-years. Cosmological distance can be defined in several ways, so this is best treated as an approximate scale rather than a single exact figure.

Why are there so few galaxies inside it?

It grew from a region that began slightly less dense than average. Over billions of years, matter moved preferentially toward surrounding filaments, walls and clusters, leaving the interior increasingly underdense.

Is the Boötes Void the largest void known?

It is one of the most famous large cosmic voids, but it is not safe to call it the single largest known. Modern surveys identify many large voids and supervoid-like structures, and the measured size depends on survey coverage and the method used to define a void.

Could the Boötes Void be evidence of aliens or an artificial structure?

There is no evidence for an artificial origin. Its large-scale properties fit the cosmic-web picture produced by gravitational growth from primordial density fluctuations.