We tend to think of the universe as roughly uniform, with galaxies scattered in every direction — but the Boötes Void, discovered in 1981 during a redshift survey, is a vast cosmic underdensity up to 330 million light-years across, containing only a few dozen known galaxies where thousands might otherwise be expected

On the scale of the whole observable universe, matter is assumed to be spread out fairly evenly: enough galaxies in enough directions that no single patch of sky should look dramatically emptier than any other. The Boötes Void is the case that does not fit that picture. It is a roughly spherical region of space, in the direction of the constellation Boötes, that has come up strikingly empty every time astronomers have gone looking for galaxies inside it.

The astronomer Robert Kirshner, working with Augustus Oemler, Paul Schechter and Stephen Shectman, first reported it in a short paper in The Astrophysical Journal Letters in 1981, drawn from a survey measuring the redshifts of galaxies in that part of the sky. A fuller survey confirming and mapping the void followed in The Astrophysical Journal in 1987.

How a redshift survey finds an empty patch of sky

Redshift is the tool that makes a finding like this possible at all. Light from a galaxy moving away from us, which almost all distant galaxies are due to cosmic expansion, is stretched toward longer, redder wavelengths, and the amount of stretching scales with distance. Measuring redshift for every galaxy in a patch of sky effectively converts a flat photograph into a three-dimensional map, with each galaxy placed at its true distance rather than just its position on the page.

When Kirshner’s team ran that conversion for the region around Boötes, a gap opened up in the map. Where a typical volume that size would be expected to contain roughly 2,000 galaxies, subsequent surveys have located only about 60. The void is not a flat photograph missing a few dots. It is a three-dimensional hole in the galaxy distribution, confirmed by measuring where things actually sit in space rather than just where they appear.

Not empty, just close to it

“Void” slightly overstates the case.

The region is not a vacuum, and the 60 known galaxies inside it are not spread randomly through the volume. Most sit along a narrow, roughly tube-shaped strand running through the middle of the void, which is one reason some astronomers, as NASA’s Blueshift blog notes, think the Boötes Void did not form as a single event but grew from several smaller voids merging, the way soap bubbles combine into larger ones as the film between them breaks down.

Its size is usually given as somewhere between 250 and 330 million light-years across, with the range reflecting genuine differences between surveys rather than sloppy measurement. A void does not have a hard edge. Galaxy density thins out gradually rather than stopping at a boundary, so where exactly the void “ends” depends on the density threshold a given study uses to define it. Its centre is estimated to lie around 700 million light-years from Earth.

Why a hole this size is not a crisis for cosmology

A gap that large in the galaxy distribution might sound like it should threaten the standard picture of how the universe is built. It does not. On the biggest scales that picture, sometimes called the cosmic web, already predicts exactly this kind of unevenness: dense filaments and clusters of galaxies threaded through with large, close-to-empty gaps, arranged this way because of how small variations in density in the very early universe grew under gravity over billions of years. Voids are not the exception to that structure. They are most of its volume.

Researchers who model this growth under the standard Lambda-cold dark matter framework find that a void of Boötes’ size and depth is consistent with, rather than a challenge to, that model. The Boötes Void was unusual mainly for being large, nearby, and detected early, at a time when large-scale surveys of galaxy redshifts were still new.

It was not unusual for existing.

What the emptiness is used for

Because the Boötes Void has so few galaxies, it functions less as a curiosity and more as a natural laboratory. Galaxies that form and evolve in near-total isolation, without the mergers, tidal interactions and crowding that shape galaxies inside dense clusters, are hard to find in large numbers. The handful inside the void give researchers a way to test how much of ordinary galaxy evolution depends on having neighbours nearby.

Voids in general also feed directly into cosmological modelling, since their sizes, shapes and how often they occur depend on the underlying physics of cosmic expansion and structure growth. Later, wider surveys have since proposed voids that would dwarf Boötes: Ryan Keenan, Amy Barger and Lennox Cowie argued in a 2013 paper in The Astrophysical Journal for what is now called the KBC Void, a proposed underdensity roughly 2 billion light-years across that would include the Milky Way’s own galactic neighbourhood.

Unlike Boötes, the KBC Void’s existence is still contested among cosmologists, with some researchers arguing a structure that large sits awkwardly against the standard cosmological model and others arguing it does not. Boötes was never the largest structure of its kind in an absolute sense. It was simply the first hole of this scale that a redshift survey happened to find, at a moment when the very existence of holes that size was still a genuine open question.