The Boötes Void is one of the largest known underdense regions in the observable universe, a roughly spherical expanse about 250 to 330 million light-years across that contains startlingly few galaxies for its size. Discovered in 1981 during a redshift survey of the constellation Boötes, it challenged astronomers’ expectations about how evenly matter should be spread across the cosmos. The void is not truly empty, but what little it contains, and what its existence tells us about the physics governing the universe’s large-scale structure, turns out to be far more interesting than the emptiness itself.
How Big the Emptiness Actually Is
To appreciate the Boötes Void, you need a sense of cosmic scale. Our Milky Way sits in a neighborhood called the Local Group, a cluster of a few dozen galaxies spanning roughly 10 million light-years. The Boötes Void could swallow dozens of Local Groups and still have room left over. Estimates of its diameter range from about 250 million to 330 million light-years, depending on how the boundary is defined and which survey data is used. One simulation study placed it at roughly twice the scale of a 10 megaparsec Gaussian-smoothed underdensity, consistent with a diameter in that ballpark.1Oxford Academic (Monthly Notices of the Royal Astronomical Society). Voids in gravitational instability scenarios – I. Global density and velocity fields in an Einstein–de Sitter universe
When Robert Kirshner, Augustus Oemler, Paul Schechter, and Stephen Shectman published their finding, the void contained only about 60 known galaxies where thousands might have been expected based on the average density of the universe. That number has grown slightly with deeper surveys, but the region remains dramatically underpopulated. If galaxies were distributed randomly, a volume that size would hold something on the order of 2,000 galaxies. The Boötes Void holds a small fraction of that. For perspective, if you stood on a planet in the middle of the void, the nearest galaxy could be tens of millions of light-years away, rather than the roughly 2.5 million light-years that separates us from Andromeda.
How Cosmic Voids Form
The Boötes Void didn’t appear by chance. It is one product of the same gravitational process that built galaxy clusters, filaments, and sheets. In the very early universe, matter was distributed almost uniformly, but tiny density fluctuations left over from the Big Bang meant some regions had slightly more matter and some had slightly less. Over billions of years, gravity amplified those differences. Denser patches pulled in surrounding material and eventually formed galaxies, galaxy groups, and clusters. The regions that started slightly underdense lost matter to their denser neighbors, becoming progressively emptier.
This process creates what cosmologists call the cosmic web: a foam-like arrangement of dense filaments and walls surrounding vast, nearly empty voids. Simulations of this structure under the standard cosmological model reproduce voids that look much like what we observe. One key finding from such simulations is that as voids evolve, small voids get absorbed into larger ones, so the number of small voids shrinks while the number of large voids grows over time.2Monthly Notices of the Royal Astronomical Society. Shapes and sizes of voids in the Lambda cold dark matter universe: excursion set approach This merging process may help explain why a void as enormous as the Boötes Void exists at all: it could be the result of several smaller voids coalescing over cosmic time.
The same simulation work also found that large voids are generally not spherical, despite being commonly depicted that way. Their boundaries tend to be irregular and shaped by the structures surrounding them. The Boötes Void looks roughly spherical in early survey data, but finer measurements suggest its shape is more complex, as is typical for voids of its scale.3Monthly Notices of the Royal Astronomical Society. Shapes and sizes of voids in the Lambda cold dark matter universe: excursion set approach Simulations of gravitational instability in void interiors also show that while the center of a void tends to become rounder over time, the outer boundary is heavily influenced by the walls and filaments enclosing it.4Oxford Academic (Monthly Notices of the Royal Astronomical Society). Voids in gravitational instability scenarios – I. Global density and velocity fields in an Einstein–de Sitter universe
Why So Few Galaxies? The Void Phenomenon
The scarcity of galaxies inside voids goes beyond what you might expect from the simple story of matter flowing out of underdense regions. Even accounting for that outflow, cosmological models predict that dark matter clumps called halos should still form inside voids, and those halos should host at least some faint, small galaxies. But observations consistently find fewer galaxies in voids than the models naively predict, particularly faint ones. This discrepancy has been called the “void phenomenon.”
For a while, this looked like it might be a genuine problem for the standard cosmological model. But more careful modeling has shown that the mismatch is smaller than it first appeared, and may not be a problem at all. One analysis demonstrated that if you assume galaxy brightness is simply a function of the mass of the dark matter halo hosting it, the standard model naturally predicts large, empty voids extending about 15 megaparsecs in diameter, even for galaxies seven magnitudes fainter than typical bright galaxies. That model matched several statistical measures of void emptiness, including the luminosity function of galaxies in underdense regions and the probability of finding voids with no galaxies whatsoever.5The Astrophysical Journal. The Void Phenomenon Explained
More recent work has explored whether modifications to the initial spectrum of density fluctuations could further resolve the tension. One study found that adjusting the initial conditions increased the likelihood of finding massive dark matter halos embedded in voids while simultaneously decreasing the number of small halos and the faint galaxies they would host. The result is that massive halos do exist in voids but are rare enough that they went unnoticed, while the small faint galaxies surveys were looking for simply never formed in significant numbers.6Monthly Notices of the Royal Astronomical Society. Addressing the too-big-to-fail problem and the void phenomenon through a modified initial power spectrum
The upshot is that the emptiness of the Boötes Void, while striking, is not evidence that something is fundamentally wrong with our understanding of cosmology. The standard model of structure formation, with reasonably straightforward assumptions about how galaxies populate dark matter halos, predicts voids like this one.
What the Few Void Galaxies Are Like
The galaxies that do reside inside voids are not identical to those in denser environments, but the differences are subtler than you might guess. One of the more extensive studies of void galaxy properties found that the specific star formation rate of void galaxies follows the same declining trend with stellar mass as galaxies in somewhat denser environments.7Monthly Notices of the Royal Astronomical Society. The void galaxy survey: Star formation properties In other words, a void galaxy and a field galaxy of the same mass are forming stars at roughly comparable rates. The void environment does not shut down star formation or dramatically accelerate it for isolated galaxies.
Where things get more interesting is when void galaxies are not alone. Galaxy pairs inside voids show significantly greater star formation efficiency compared to pairs in denser wall and field environments. This may be because the gas reservoirs around void galaxies tend to be richer and less disturbed. In denser environments, galaxies interact more violently, stripping each other of gas through strong tidal forces and high-speed encounters. Inside a void, galaxy interactions are gentler, and the orbits of interacting pairs are less eccentric, which allows the gas to be funneled toward star-forming regions more efficiently.8Astronomy & Astrophysics. Galaxy pairs in cosmic voids
Simulations of how void galaxies assemble their structures support this picture. Studies of galaxies forming along thin filaments inside voids find that most have quiet merger histories, experiencing only minor mergers over the latter half of cosmic time. The occasional system that does undergo a major merger tends to do so quite late, assembling its dominant structure from many smaller progenitors rather than through a single dramatic collision.9Oxford Academic. Assembly of filamentary void galaxy configurations
Even the morphologies of void galaxies tell a story of quiet, internal evolution. A recent study of over 300 ring galaxies inside voids found that the rings are predominantly inner rings and pseudorings formed through secular processes, meaning slow internal dynamics like bar-driven gas flows, rather than violent collisions. Compared to the general void galaxy population, ring galaxies in voids tend to be more massive, redder, and have lower specific star formation rates, suggesting they represent a more evolved subset shaped primarily by their own internal dynamics rather than by their sparse surroundings.10arXiv. Environmental Dependence of Galaxy properties: A study of 341 Ring Galaxies in Cosmic Voids
What Fills the Void Besides Galaxies
The Boötes Void and voids like it are not truly empty. They contain diffuse gas, dark matter at very low density, and faint magnetic fields. The gas is detectable through its absorption of light from more distant objects. When light from a distant quasar passes through a void, certain wavelengths get absorbed by hydrogen gas along the way, producing features called Lyman-alpha absorption lines. A large survey using Hubble Space Telescope observations found that about 65 percent of low-redshift Lyman-alpha absorbers fall inside cosmic voids, which is close to the volume fraction that voids occupy (about 68 percent). By contrast, only about 21 percent of galaxies live inside voids.11arXiv. HST /COS Lyman-alpha Absorbers in Cosmic Voids
This tells us something important: while galaxies are heavily concentrated along filaments and walls, diffuse gas is spread much more evenly throughout voids. Inside voids, galaxies cluster near the edges, but the gas is distributed relatively uniformly. The study also found evidence for two distinct populations of absorbers: lower column density systems that are nearly uniformly spread through void interiors, and higher column density systems that cluster near galaxies at void edges.12arXiv. HST /COS Lyman-alpha Absorbers in Cosmic Voids So the void interior is not a perfect vacuum. It is a thin, warm bath of hydrogen gas, with occasional denser patches near whatever galaxies exist along the void’s periphery.
Voids also carry magnetic fields, though extremely weak ones. Observations of blazars, intensely bright active galaxies, provide a way to set lower limits on these fields. When high-energy gamma rays from a blazar interact with background infrared light, they produce electron-positron pairs, which in turn emit secondary gamma rays. If an intergalactic magnetic field is present, it deflects these pairs, altering the timing and direction of the secondary emission in ways that telescopes can detect. Blazar observations imply that cosmic voids are magnetized at a level of at least about 10^-17 gauss on megaparsec scales.13Physical Review D. Can galactic magnetic fields diffuse into the voids? Stricter limits from long-term monitoring of the blazar Mrk 421, whose light passes through a large intergalactic void mapped by the Sloan Digital Sky Survey, exclude magnetic field strengths below about 10^-20.5 gauss for a field coherence length of 1 kiloparsec.14The Astrophysical Journal Letters. LOWER BOUNDS ON MAGNETIC FIELDS IN INTERGALACTIC VOIDS FROM LONG-TERM GeV–TeV LIGHT CURVES OF THE BLAZAR MRK 421
These field strengths are fantastically small compared to everyday magnets or even Earth’s magnetic field, but their mere existence is scientifically significant. The origin of these void magnetic fields is still debated. They could be relics of processes in the very early universe, or they could have been seeded by outflows from galaxies and active galactic nuclei and then spread into the void interiors over cosmic time.
Voids as Tools for Measuring the Universe
Beyond being curiosities of cosmic geography, voids have become surprisingly useful instruments for testing fundamental physics. Their simple geometry and low density make them cleaner laboratories than the crowded environments of galaxy clusters for certain cosmological measurements.
One powerful technique uses the shapes of stacked voids to test whether the universe is expanding as the standard model predicts. This is called the Alcock-Paczyński test. The idea is that if you assume voids are statistically spherical (which stacking many voids together ensures), any observed elongation or compression of the stacked shape must be due to an incorrect assumption about the expansion rate. A study using about 1,500 voids identified in Sloan Digital Sky Survey data found substantial evidence for this signal, with results favoring a matter density parameter of about 0.15 over a matter-dominated universe by a likelihood ratio of 10, and preferring the standard dark-energy model over no signal by a factor of about 4.5.15Monthly Notices of the Royal Astronomical Society. A measurement of the Alcock–Paczyński effect using cosmic voids in the SDSS The technique has matured considerably and is now recognized as competitive with more traditional methods of constraining dark energy, especially when combined with galaxy clustering measurements.16Astronomy & Astrophysics. Alcock–Paczyński effect on void-finding: Implications for void-galaxy cross-correlation modelling
Voids also leave a subtle imprint on the cosmic microwave background, the ancient light left over from the early universe. As this light passes through a void, it gains a tiny amount of energy if the void is expanding faster than the surrounding matter is contracting, an effect related to dark energy’s influence on cosmic expansion. This is called the integrated Sachs-Wolfe effect. Recent work cross-correlating large void catalogs with temperature maps from the Planck satellite achieved one of the highest-significance detections of this signal ever reported from a single catalog, reaching 3.6 sigma.17arXiv. Cosmic Tunnels and the Integrated Sachs-Wolfe effect This is not a marginal detection; it represents genuine evidence that voids interact with the cosmic microwave background in the way dark energy models predict.
Voids can even be used to test theories of modified gravity. Weak gravitational lensing, the subtle bending of light from distant galaxies by intervening mass, works differently in underdense regions than in overdense ones. Two-dimensional voids detected in weak lensing maps trace underdense regions along the line of sight and are sensitive to the unbiased distribution of matter, making them a direct probe for testing whether gravity behaves as general relativity predicts on the largest scales.18Astronomy & Astrophysics. Weak-lensing tunnel voids in simulated light cones: A new pipeline to investigate modified gravity and massive neutrinos signatures
Pushing Deeper Into Voids With New Surveys
Much of what we know about voids comes from relatively nearby regions of the universe, simply because detecting faint galaxies or diffuse gas at great distances is hard. But that is changing. The Euclid space telescope, launched in 2023 by the European Space Agency, will collect spectroscopic data for tens of millions of galaxies over 15,000 square degrees of sky in the redshift range of roughly 0.9 to 1.8, corresponding to when the universe was about half its current age. Forecasts indicate that measuring the cross-correlation between voids and galaxies in the Euclid survey will provide tight constraints on cosmological parameters, including the equation of state of dark energy.19Astronomy & Astrophysics. Euclid: Cosmology forecasts from the void-galaxy cross-correlation function with reconstruction
Meanwhile, three-dimensional mapping of voids using the absorption of light by intergalactic hydrogen is opening a window to much earlier times. One team used spectra from 240 background galaxies and quasars to build a tomographic map of the intergalactic medium in a small patch of sky, and identified cosmic voids at a redshift of about 2.3, the most distant voids ever detected at the time. The volume fraction of voids in the observed data (about 19.5 percent) closely matched simulations (about 18.2 percent), confirming that the technique is reliable.20The Astrophysical Journal. Detection of z ∼ 2.3 Cosmic Voids from 3D Lyα Forest Tomography in the COSMOS Field Extending this approach to wider fields with upcoming instruments could eventually map the void network across most of cosmic history, revealing how voids have grown and merged over time.
The Boötes Void itself remains the archetype of cosmic emptiness, but the science it catalyzed has expanded far beyond one particular hole in the galaxy distribution. Voids turn out to be surprisingly rich environments for studying the physics of the universe: less cluttered, more geometrically tractable, and more sensitive to the effects of dark energy and gravity than the crowded filaments and clusters that tend to dominate astronomical attention.