The universe is a bit like a hunk of Swiss cheese. Matter tends to clump together into giant structures known as clusters and superclusters, with big empty "bubbles" between them.
The bubbles are known as voids, and they can span millions of light-years. They have few galaxies, and little "dark matter" -- matter that produces no detectable energy, but that exerts a gravitational tug on the visible matter around it.
The largest void yet discovered spans a billion light-years. It contains almost no matter of any kind. By comparison, if you carved out a similar volume of space around our Milky Way galaxy, it would include millions of other galaxies, plus massive amounts of dark matter.
Astronomers learned of the void a few years ago. A satellite that mapped the "afterglow" of the Big Bang found that a large region of space was unusually cold -- the signature of a void.
And two years ago, a survey of that region by an array of radio telescopes found almost no galaxies there.
The void is much larger than any other yet detected. It's so big, in fact, that no one can quite explain it -- it just doesn't seem possible that so large a volume of space could be so bereft of any form of matter.
This great galactic void spans a large part of the constellation Eridanus, which meanders across the southwest this evening. The void is not only a big hole in the universe -- it shows that there are still big voids in our understanding of the universe, too.
Script by Damond Benningfield, Copyright 2008
For more skywatching tips, astronomy news, and much more, read StarDate magazine.