Astronomy often focuses on where things are—stars, galaxies, clusters—rather than where they aren't. However, sometimes emptiness can teach scientists just as much. A spherical region of space 250 million light years in diameter, the Boötes void (pronounced boo-OH-tees) is the emptiest area of space in the known universe. In all that space, the void contains only 60 galaxies. For comparison, as io9 points out, our own galaxy has around two dozen galactic neighbors in a space of only 3 million light years. Given that the average distance between galaxies everywhere else in the universe is a few million light years, an expanse the size of the Boötes void should contain around 10,000 galaxies. It contains only 0.6% of that number.
There are several theories for the void's existence. One is that galaxies have a tendency to gravitate toward each other, leaving areas of empty space behind. But because the universe hasn't been around long enough for this to create a space the size of the Boötes void, another theory takes an opposite approach: perhaps it's the voids, not the galaxies, that are coming together to create a larger void. Of course, no strange astronomical phenomenon would be complete without an alien theory. It could be that the void does contain galaxies, but the stars within them have been blanketed by a Dyson shell used to power a super-advanced civilization. Which theory is the most plausible? Watch the videos below to form your own opinions.