Bootes Void

The Bootes Void is a tremendously large, nearly-empty region of space, devoid of galaxies. At nearly 250 million light-years across, it is one of the largest known voids, and is referred to as a supervoid.

To give an idea of its scale, "If the Milky Way had been in the center of the Bootes void we wouldn't have known there were other galaxies until very recently in our history."

It has been theorized that the Bootes void was formed from the merger of smaller voids, much as soap bubbles coalesce to form larger bubbles. This theory would account for the small number of galaxies that exist in a rough tube shape through the middle of the void.

It has also been theorized that the Bootes void isn't empty, and is instead filled with dark matter.