The Boötes void is a bubble of near-nothing about 330 million light-years across

In the direction of the constellation Boötes lies one of the emptiest regions ever mapped in the universe. Known as the Boötes void, it is a roughly spherical stretch of space about 330 million light-years across in which astronomers have found only a scattering of galaxies where thousands would be expected. It is so devoid of matter that if the Milky Way sat at its center, humanity might not have discovered other galaxies until well into the twentieth century.

A hole hundreds of millions of light-years wide

The void’s scale is difficult to grasp. At about 330 million light-years in diameter, it spans a significant fraction of a percent of the observable universe’s width, making it one of the largest known voids, sometimes called a supervoid. Its center lies roughly 700 million light-years from Earth, placing the entire structure deep in the distant cosmos rather than in the Milky Way’s immediate neighborhood.

Within that enormous volume, surveys have turned up only around 60 galaxies. Based on how galaxies are typically distributed, a region that size should contain on the order of 2,000. The reference record for the Boötes void summarizes those figures and the calculations behind them, underscoring how sparsely populated the region is compared with the cosmic average.

Discovered by accident in a redshift survey

The void was identified in 1981 by a team of astronomers led by Robert Kirshner during a survey measuring the redshifts of galaxies, a technique that gauges distance by how much a galaxy’s light is stretched toward longer wavelengths. As the researchers charted where galaxies sat along their line of sight, they found a broad gap where galaxies simply were not, a conspicuous absence rather than a cluster.

That kind of emptiness was startling at the time because the universe was often pictured as more evenly filled with galaxies. The discovery of such a vast hole helped drive home that matter is not spread uniformly across space. Popular explainers of the Boötes void, such as one from a science magazine account of the region, trace how the accidental find reshaped thinking about the structure of the cosmos.

Near-nothing, but not truly empty

The word void can be misleading. The Boötes void is not a perfect vacuum or a region of literally zero galaxies; it contains dozens, some of which form a loose, roughly tube-shaped chain running through the interior. What defines the void is its extreme underdensity, meaning it holds far fewer galaxies per unit of volume than surrounding space, not a complete absence of them.

Even so, the galaxies inside are so spread out that any inhabitants of one would see a strikingly dark sky. Neighboring galaxies would be tens of millions of light-years away in every direction, far more isolated than galaxies in a denser region like the one the Milky Way occupies. That isolation is the source of the frequent description of the void as a bubble of near-nothing.

Voids as part of the cosmic web

The Boötes void is not an anomaly so much as an extreme example of a general pattern. On the largest scales, matter in the universe is arranged in what astronomers call the cosmic web, a network of filaments and sheets of galaxies surrounding enormous empty spaces. Galaxies cluster along the strands of this web, and the gaps between the strands are voids.

NASA’s material on the structure of the universe describes how this web emerged as gravity gradually pulled matter together over billions of years, drawing it out of some regions and concentrating it in others. In that picture, voids are the natural counterpart of clusters: as matter flows toward dense filaments, the spaces it leaves behind grow ever emptier.

How a void this large may have formed

One leading idea for the Boötes void’s origin is that it formed from the merging of several smaller voids, much as small soap bubbles can join into a single larger one. Under this scenario, a number of moderately empty regions expanded and coalesced over cosmic time into one immense low-density zone, which would help explain the ragged chain of galaxies threading its interior.

Because voids expand as the universe grows and as gravity continues to sweep matter toward denser structures, a supervoid represents the long-term outcome of these processes acting on a large scale. Studying its size, shape, and the galaxies it does contain gives cosmologists a way to test models of how structure develops across the universe.

What emptiness reveals

Far from being uninteresting, regions of near-nothing are valuable tools for cosmology. The abundance, size, and distribution of voids depend on the underlying properties of the universe, including how matter clumps and how cosmic expansion behaves. Measuring giant voids like the one in Boötes helps researchers constrain those properties and check whether their models of the cosmos hold up.

The Boötes void endures as a vivid demonstration that the universe is mostly empty space punctuated by concentrations of galaxies, rather than a smoothly filled expanse. Hundreds of millions of light-years wide and nearly barren, it stands as one of the clearest windows into the vast, structured emptiness that lies between the galaxies.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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