Voids & Supervoids in the Universe

Cosmic voids are large volumes of space, tens of megaparsecs or more across, that are devoid of matter. This includes normal matter, in the form of galaxies, and dark matter. Initially, astronomers were not sure if the voids contained dark matter, even though there were no galaxies, but recent observations show that the halos of dark matter are not present. The filamentary structure of galactic superclusters surrounds the voids. The largest confirmed supervoids are 100 to 150 Mpc across (close to a million light-years). To explain the cold spot in the CMB, some astronomers propose a huge supervoid, tentatively dubbed the Eridanus supervoid, and this would be around 280 Mpc across. Note that the cold spot is only about 70 millionths of a degree colder than the rest of the background. Prior to this, one of the largest known voids was the Boötes Supervoid at around 80 Mpc in diameter. It is interesting in that it has a "tube" like area containing a small number of galaxies that runs through it, leading to the hypothesis that it formed as the merger of a number of smaller voids. The Northern and Southern Local Supervoids are slightly larger than the Boötes Supervoid. The Capricornus void is also disputed, but would be around 230 Mpc across. This link has some of the latest information on cosmic voids (you need to scroll down to find the article).

Science - Large Scale Structure of the Universe

Voids & Supervoids