How can there be a void within a void? Space is generally considered the ultimate void: after all, normal matter only makes up between 1 and 10% of the known universe, so automatically, the rest should be void. But it turns out that some parts of space are just a little too empty, a little too underpopulated. Voids come in varying sizes, and generally are discovered via “cold spots” in the map of cosmic microwave background (CMB) charted by astronomers across the known universe. Little pockets of nothing are not that strange, but big pockets of the known universe where there are few if any stars are another thing entirely.
The largest void theorized to exist is actually very close: the KBC void, also known as the local hole, is theorized to house the Milky Way galaxy, along with our local cluster. If the observations of the scientists it’s named after (Ryan Keenan, Amy Barger, and Lennox Cowie) are correct, our galaxy is in a hole that may be anywhere from 1 to 2 billion light-years across, situated a few hundred million lightyears away from the center. The Boötes void, discovered in the 1980s, is approximately 250 million lightyears across, seems to have only about 60 galaxies in it. For comparison, our own galaxy (which as noted before is in its own rather large hole) has over two dozen galactic neighbors.
Why are these patches of nothing important? For one thing: they challenge our existing ideas of the formation of the universe and just how old our universe is. Even the billions of years we know the universe hasn’t existed shouldn’t have been long enough for regular spreading to create giant holes of nothing. Which leads to the other reason: the voids could, some scientists theorize, be created by surges of dark energy. Ultimately, however, nobody knows why the universe seems to be dotted with these random, enormous holes with little to nothing visibly going on in them.