Space & Cosmic

The Bootes Void: How Astronomers Found a 330-Million-Light-Year Hole in the Universe

By The Unsolved Report Editorial Team · Last reviewed 2026-06-19

The Bootes Void is a 330-million-light-year region nearly empty of galaxies. Here is what the data actually shows and why it does not break cosmology.

In 1981, a team of astronomers mapping the distances to galaxies expected to find more of the same: galaxies scattered fairly evenly across the sky, the way stars are scattered across a clear night. Instead, when they looked toward the constellation Bootes (the Herdsman), they found almost nothing. A region of space roughly 330 million light-years across held a small fraction of the galaxies it should have. The team had stumbled onto one of the emptiest known places in the universe, now called the Bootes Void.

The story is often told with a famous line attributed to the astronomer Greg Aldering, used to convey the scale of the emptiness: if the Milky Way had been in the center of the Bootes Void, we would not have known there were other galaxies until the 1960s. Whether or not the quote is exact, the point stands. The void is so large and so empty that a civilization inside it could have spent decades believing its own galaxy was alone.

How the void was discovered

The discovery came from a redshift survey led by Robert Kirshner and colleagues, then working to build a three-dimensional map of the local universe. A galaxy's redshift - how much its light is stretched toward longer wavelengths by cosmic expansion - serves as a proxy for its distance. By measuring redshifts for many galaxies, astronomers can reconstruct where those galaxies sit in depth, not just where they appear on the flat sky.

When Kirshner's team plotted their results in the direction of Bootes, a vast under-dense region appeared. The 1981 paper in The Astrophysical Journal, titled "A million cubic megaparsec void in Bootes," reported the find. The discovery was partly accidental, a byproduct of a survey designed for other purposes, which is part of why it is remembered as a genuine surprise rather than a confirmation of theory.

Follow-up work in the years after refined the picture. Rather than being perfectly empty, the void turned out to contain a modest population of galaxies, many of them arranged in a tube-like filament threading through the middle. Estimates put the count at roughly 60 galaxies inside a region where standard galaxy density would predict closer to 2,000. So the Bootes Void is not a true vacuum. It is a region where galaxies are extraordinarily scarce, on the order of one-tenth or less of the cosmic average.

Just how big and how empty

A few numbers help anchor the scale:

Diameter: about 330 million light-years across, making it one of the largest known voids (some sources describe it as a "supervoid").
Distance: roughly 700 million light-years from Earth, in the direction of the constellation Bootes.
Galaxy count: on the order of 60 confirmed galaxies inside, versus the ~2,000 a comparable populated volume would hold.

It is worth being precise about what "empty" means here. The void is under-dense, not void of all matter. It still contains diffuse gas, dark matter, and the handful of galaxies noted above. The striking thing is the contrast with its surroundings, not an absolute absence of stuff.

Why voids exist at all

Here is the crucial point that often gets lost in dramatic retellings: the existence of large voids is not a mystery. It is a prediction. Modern cosmology expects the universe to look like this.

In the standard model of cosmology, known as Lambda-CDM, the universe began nearly uniform, with only tiny density fluctuations imprinted in the very early universe. Over billions of years, gravity amplified those fluctuations. Slightly denser regions pulled in more matter and grew denser still, while slightly under-dense regions lost matter to their neighbors and grew emptier. The end result is what astronomers call the cosmic web: a spongelike structure of dense filaments and sheets of galaxies surrounding enormous, nearly empty bubbles.

Voids are the bubbles. They are not anomalies in this picture. They are the natural complement to the filaments, the empty space between the strands of the web. Cosmological simulations that start from the early universe and let gravity run forward reliably produce voids, including very large ones.

So why does the Bootes Void still get attention? Partly its sheer size puts it toward the large end of what models comfortably produce. A void this big is not impossible under Lambda-CDM, but it is uncommon, and its existence is a useful test of how well our structure-formation theory matches reality. Studying the largest voids helps calibrate the models.

Established versus open

It is worth separating what is solidly known from what remains uncertain.

Established:

The Bootes Void is real, repeatedly confirmed by multiple surveys since 1981.
It is severely under-dense, with far fewer galaxies than average.
The general existence of large cosmic voids is well explained by gravitational growth of structure in Lambda-CDM.

Still discussed:

Whether the Bootes Void's size is fully typical for the standard model, or sits at the rare tail of the distribution.
The detailed properties of the few galaxies inside, including whether void galaxies form stars differently because they evolve in relative isolation.

That last question is genuinely interesting and is active research. Galaxies that grow up inside voids have few neighbors to interact or merge with. Studies using large surveys such as the Sloan Digital Sky Survey have looked at whether void galaxies are bluer, more gas-rich, or forming stars at different rates than galaxies in denser environments. The differences found are real but generally modest, suggesting that environment shapes galaxies somewhat but does not rewrite their basic nature.

What the void is not

Because the Bootes Void is dramatic, it sometimes attracts overreach. A few clarifications:

It is not a black hole or a tear in space. It is simply a region with very little matter. Nothing is being swallowed.
It is not evidence of alien engineering. There is no scientific basis for that idea, and nothing about the void requires an exotic explanation. Ordinary gravity acting over billions of years accounts for it.
It does not break physics. If anything, large voids are confirmation that our broad picture of cosmic structure formation works.

The honest mystery, to the extent there is one, is quantitative rather than qualitative. The question is not "how can a void exist?" but "is a void this large fully consistent with the statistics our best models predict?" That is a question of fine detail, the kind cosmologists test by comparing many voids across large surveys against simulations.

How astronomers map emptiness

It may seem strange to speak of measuring a region defined by what is not there, but cosmologists have developed careful tools for exactly this. The first ingredient is a galaxy survey: a catalog of positions and redshifts for hundreds of thousands or millions of galaxies. Projects such as the Sloan Digital Sky Survey and the earlier galaxy redshift surveys turned the two-dimensional sky into a three-dimensional map, with redshift standing in for depth.

Once you have that map, void-finding algorithms hunt for regions where the galaxy density drops well below the cosmic average. Different algorithms define voids slightly differently - some grow spheres outward from empty points until they bump into galaxies, others use the geometry of the galaxy distribution itself - which is one reason quoted void sizes can vary between studies. The Bootes Void's headline figure of roughly 330 million light-years reflects this kind of analysis applied to a strikingly under-dense region.

A subtlety worth noting: redshift is not a perfect distance measure. Galaxies have their own motions on top of the overall cosmic expansion, which smears their apparent positions in depth slightly. Astronomers account for these "redshift-space distortions" when characterizing voids, and the corrections are well understood. None of them make the Bootes Void disappear; they refine its shape and boundaries.

Voids across the cosmic web

The Bootes Void is the most famous, but it is far from alone. The universe is riddled with voids of many sizes, and together they make up most of its volume. Galaxies, by contrast, are concentrated into the thin walls and filaments between voids. If you could zoom out far enough, the cosmos would look less like a sea of galaxies and more like a froth of bubbles, with galaxies tracing the soap films between them.

This froth-like architecture is precisely what gravity acting on small early-universe fluctuations is expected to produce. Other very large under-dense regions have been catalogued in different parts of the sky, and surveys continue to find more. The point is that voids are a generic, expected feature, and the Bootes Void is best understood as a particularly large and well-studied example of an entirely ordinary phenomenon rather than a unique enigma.

Why it matters

Voids are not just curiosities. They are becoming serious cosmological tools. Because voids are under-dense, the way light and matter behave inside them is sensitive to the properties of dark energy and to the laws of gravity on the largest scales. The expansion history inside a void differs subtly from the cosmic average, and the gravitational potential a void carves out can leave a faint imprint on the cosmic microwave background as light passes through.

That last effect connects the study of voids to other frontier questions in cosmology, including anomalies in the microwave background itself. Mapping voids in detail, and comparing their abundance and sizes to predictions, gives an independent check on the model that also rests on supernovae, the microwave background, and galaxy clustering.

The Bootes Void, found by accident in 1981, turned out to be an early window onto the true architecture of the cosmos: not a uniform sprinkling of galaxies, but a vast, structured web wrapped around enormous bubbles of near-emptiness. That architecture is one of the genuine triumphs of modern cosmology, and the void in Bootes was one of the first dramatic hints that the universe is built that way.

Sources & further reading

Wikipedia - Bootes Void - https://en.wikipedia.org/wiki/Bo%C3%B6tes_Void
Kirshner et al. 1981, A million cubic megaparsec void in Bootes, The Astrophysical Journal - https://ui.adsabs.harvard.edu/abs/1981ApJ...248L..57K/abstract
NASA/IPAC Extragalactic Database - The Void in Bootes - https://ned.ipac.caltech.edu/level5/Bothun2/Bothun3_5_3.html
BBC Sky at Night Magazine - The Bootes Void - https://www.skyatnightmagazine.com/space-science/bootes-void
Wikipedia - Void (astronomy) - https://en.wikipedia.org/wiki/Void_(astronomy)
Wikipedia - Lambda-CDM model - https://en.wikipedia.org/wiki/Lambda-CDM_model