In 1981, astronomers stumbled onto what looked, at the time, like the largest hole ever found in the universe. It didn’t turn up in a telescope. It turned up on a bar chart.

Four astronomers, Robert Kirshner, Augustus Oemler Jr, Paul Schechter and Stephen Shectman, published a four-page letter in The Astrophysical Journal with a question mark in the title. Between them they had measured redshifts for 133 galaxies across three small patches of northern sky. Each patch was about a degree and a half wide, and each sat some 35 degrees from the other two. Redshift stands in for distance, so plotting those 133 numbers gives you a crude map of depth. Their plot should have humped up in the middle, where the survey was most sensitive. Instead, it sagged. In the 6,000 kilometre-per-second band centred on 15,000, only one of the 133 galaxies was sitting there.

Their own maths had asked for about twenty-five.

Doubts from the authors themselves

The letter asks it outright: was the histogram “a profound fact about the universe or a statistical accident”? Kirshner and his co-authors had their doubts too. They noted that they had drawn the boundaries of the gap after seeing where the points fell, which flatters any probability you calculate afterwards. Their own supporting evidence, by their own admission, was slim. Three narrow beams had been sampled, and the claim on the table was that the enormous wedge of space between those beams was empty too.

Nor were they the first to find a hole. Laird Thompson and Stephen Gregory reported voids of their own in 1978, the same year a separate Estonian team found some independently. They later placed the Boötes work in the second wave of redshift surveys.

What the 1987 follow-up found

The follow-up settled it. Kirshner’s group went back and measured 240 bright galaxies in the region between their three original fields, and not one had a redshift between roughly 12,000 and 19,000 kilometres per second. A uniformly populated universe predicts 31 galaxies in that slice, as documented by the astronomer Greg Bothun in a review of the observations hosted by Caltech’s extragalactic database.

The same survey shrank the void’s claimed size. Thompson and Gregory put the remeasured volume at about a third of the figure advertised in the 1981 title. That leaves a radius near 34 megaparsecs per unit of the Hubble parameter, which lands somewhere around 150 million light-years depending on whose expansion rate you prefer. Modern estimates of the diameter run from 250 to 330 million light-years, and the centre sits roughly 700 million light-years from here. BBC Science Focus makes the scale legible: the Milky Way would fit inside the region billions of times over.

Sixty galaxies, arranged in a tube

Astronomer Greg Aldering supplied the line that made the void famous. From inside it, he said, we “wouldn’t have known there were other galaxies until the 1960s”. Edwin Hubble sorted that out in the 1920s, so Aldering is docking us four decades of cosmology.

His comparison appears in a post on NASA Goddard’s Blueshift blog. The post runs the sum behind the headline number too. Take one galaxy per ten million light-years as a rough density for the general universe, apply it to this volume, and you should find about 2,000. Surveys have found around 60. That figure comes from a rule of thumb, so treat it gently, but even reasonable fiddling with the assumptions doesn’t move the ratio much.

The 60 are not scattered evenly, either.

They sit along a rough tube running through the middle, which is one reason astronomers suspect the region was assembled from smaller voids that merged, the way soap bubbles combine when the film between them gives out.

How Aldering’s own data made the void ordinary

Does the emptiness amount to an anomaly? Aldering spent years combing infrared satellite data for anything hiding in there. Bothun’s review records the haul: 53 galaxies inside the boundaries Kirshner’s team had drawn, plus a redefined density profile. That left the region’s overall characteristics looking fairly typical of voids turning up in other surveys. Aldering was the one who made the void sound so strange in the first place. He also helped show that it is a fairly ordinary void, just an unusually large one.

Entry number 88

Last year, a group led by Rosa Malandrino at the Institut d’Astrophysique de Paris published a Bayesian catalogue of 100 high-significance voids in the nearby universe. They built it by running a void-finding algorithm across fifty simulated reconstructions of the cosmic neighbourhood, then keeping only the underdensities that showed up again and again. Boötes made the cut. It is entry 88, radius 39.5 megaparsecs over h, plus or minus 2.9.

Voids fill most of the volume of the cosmos, and because they hold so little matter, their growth is governed almost entirely by cosmic acceleration. That makes them useful for testing dark energy and gravity at the largest scales. All of that leaves the Boötes Void in a strange position. Its emptiness is the ordinary condition of the universe, sampled faithfully across three hundred million light-years. We are the anomaly, perched in a bright crowded filament, doing our sums as though that were the normal way to live.