A Brief History of the Discovery of Cosmic Voids

At first the sum total of large, orderly structure in the Universe appeared to arrive in two categories. There were the clusters of galaxies – an unoriginal but descriptive name – each a dense ball with anywhere from a few dozen to a few hundred galaxies, all bound together by their mutual gravitational embrace. And then there were the field galaxies, lonely wanderers set apart and adrift from the clusters, not bound to anyone but themselves. That was it: the clusters of galaxies, the field galaxies, and the megaparsecs of emptiness that enveloped them all.

But technology is technology and advancement is advancement. The telescopes grew more powerful. The field of cosmology involved more people. The techniques improved. The development of image amplification systems – the distant forerunners of your smartphone camera – allowed astronomers to peer ever further into the dark. With every new survey taken, the number of galaxies in our Universe increased. With every night of observation, our window into the cosmos widened.

By the early 1960’s, astronomers began to realize that there was more in the Universe than mere galaxies and clusters. There was something larger – the supercluster. It only took a small sample of galaxies to reveal the shape of the first known supercluster, the Local Supercluster, with the galaxies themselves – each one the mass of a trillion suns – reduced to a tiny dot of light, acting as mere tracers of the vast structure that stretched for a million parsec on a side. The faint sketches that they could produce revealed that galaxies clump into clusters, and clusters clump into superclusters, the beginnings of our understanding of the large-scale structure of the Universe.

Time passed. Observations pushed on, finding galaxy after galaxy and cluster after cluster. Until one day, they didn’t. One mapping of the cosmos produced an unexpected result. Once again this was a survey of galaxies, clusters, and superclusters. Once again this took only a small number of galaxies to reveal the grand structure of the cosmos. Once again astronomers were intent to find the pattern, the hidden meaning in this grand design. Once again we were going to map the heavens and make it ours. But where a fair sampling of galaxies should have revealed yet more distant dots of light, there was…nothing.

A blank space.

It was a cosmic accident. A blight in the Universe. A Sahara too vast to describe except with the reducing, almost meaningless jargon of the astronomer – an empty patch devoid of galaxies almost 20 megaparsec, or 65 million light-years, across.

In 1978, we found a silence among the stars: our first cosmic void.