In 1981 Robert Kirshner and his colleagues were measuring the distances of galaxies in a patch of sky in the constellation Boötes when they noticed that a huge region seemed to have almost none. Their paper had a matter-of-fact title: “A million cubic megaparsec void in Bootes”.
How empty?
The Boötes Void has a radius of about 62 megaparsecs, nearly 200 million light-years. In a typical piece of the universe that size, astronomers would expect to find around 2 000 galaxies. In the Boötes Void, only about 60 have been found.
Why it is there
On the largest scales, matter in the universe is arranged like a sponge: galaxies gather into walls and long filaments, the cosmic web, with vast, thinly populated voids between them. Over billions of years, gravity pulls matter out of the emptier regions and onto the filaments, so the voids grow emptier still. The Boötes Void is one of the largest known and is often called a supervoid. It may have formed when several smaller voids merged. Its existence does not appear to conflict with the standard model of cosmology.
What it would be like
If the Milky Way had happened to sit in the middle of the Boötes Void, our night sky would still be full of our own galaxy’s stars. But the rest of the universe would be almost invisible: astronomers might not have known for a long time that other galaxies existed at all.