In layman terms, large voids like the Boötes void are formed by the merger of smaller neighbouring voids after surrounding galaxies merge or form their own cluster, over a period of time.
Usually galaxies visible in a void are concentrated near the walls/boundaries of these co-joined voids, which explains the galaxies leaving their original “home” to form a cluster scenario.
Just to give you an idea of how desolated the galaxies of the Boötes void actually are -
The area surrounding our Milky Way galaxy (at the very bottom) has 22+ galaxies within a span of 3 million light years. Whereas the Boötes void (area enclosed within the red circle) which has a diameter of ~ 300 million light years supposedly has only 60 observable galaxies. This much area around Milky Way galaxy should contain, on an average, at least 10,000 galaxies.
Imagine calling for help to a distant galaxy and receiving no reply as the nearest galaxy would be at least ~ 25 million light years away. You’d never want be here, so lonely, so stranded!
The Boötes void has, what we may call, the perfect vaccum in space.
There are perhaps some other speculations surrounding the Boötes void like -
It might have a Kardashev Scale Type III civilization who are capable of harnessing the entire energy of their galaxy and who are in possession of such a technology so that they could tap the entire energy and eat up neighbouring galaxies so as to fuel their growth. This comes because of the exact spherical shape of the Boötes void which indicates an intelligent species that might have created this superficial boundary of their empire.
If the Milky Way were at the centre of the Boötes Void, astronomers would not have been able to observe any other galaxies until the 1960s.
But perhaps the most interesting fact is that if you were placed smack bang in the middle of the void in a spacesuit, the nearest galaxy or light source would be so so so tremendously far away, that everything around you would be eternally black, up, down, left right, wherever you look regardless of orientation. Try imagining that. I get nightmares just thinking about it. Eternal darkness for millions of light years in every direction.
Stare into the night sky and you can’t help being amazed by the sheer scale of the universe. Look for Sirius, the brightest star in the sky. That’s 8.6 light years away. Polaris, the North Star, sits 431 light years from us, and the faintly visible Andromeda galaxy lies 2.6 million light years from Earth. These are distances that boggle the mind, yet we’re only talking about the scenery in our cosmic backyard.
Is this magnificent view typical, the sort of spectacle you’d see from anywhere in the universe? Not at all. From the middle of the Boötes Void, for instance, the universe appears a very different, and much darker, place.
The Boötes Void is a giant hole in the universe some 350 million light years across, a place where galaxies, for the most part, never formed. It lies about a billion light years from Earth in the direction of the constellation Boötes, after which it was named when it was discovered in 1981 by Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and colleagues.
What makes this expanse of nothingness so interesting? The very existence of the Boötes Void is a mystery, one that is challenging our standard story of the universe’s past. The pattern of galaxies and voids is thought to arise from tiny quantum fluctuations in the fabric of space-time at nearly the beginning of time that have been stretched to galactic proportions as the universe expanded.
These fluctuations corresponded to variations in the density of matter throughout space, and led to the formation of clusters of galaxies as well as voids between them. Yet given the age of the universe and its rate of expansion, there has only been enough time for galaxies and voids to form on a scale of tens of millions of light years – not hundreds (New Scientist, 10 March, p 30). Boötes is just too big to fit this picture.
So how did this so-called supervoid come to be? One theory, which recent evidence may back up, is that it formed when two or more ordinary voids collided. Boötes is not quite empty: it turns out to contain a sprinkling of galaxies arranged in a tubular shape. This could be the remains of the shared edge of two smaller voids before they combined.
Another possibility is that our existing theory of the big bang will have to be modified to accommodate Boötes’s existence. Both gravity and quantum mechanics played a role in forming the supervoid, but no one has figured out how to combine them into a “theory of everything”. Perhaps when we have a theory of everything, we will understand this piece of nothing, too.
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