The Eridanus supervoid is a void, a complete absence of matter, or more specifically, and absence of galaxies. Because of how matter coalesced and dispersed in the early days of the universe, there are many voids around today between galaxies and galactic clusters, but the Eridanus supervoid is unique because of its massive size. This supervoid happens to be around one billion light years across. It's massive. In 2007 it was deemed the largest void, and it still holds that title.
Because it is so phenomenally large, a few crazy ideas have been thrown around about the origin and nature of this supervoid, on of the more notable being that it was created by quantum entanglement between our galaxy and another. Now, I won't say that I believe such theories, but it's still an intriguing idea.
Another theory states that the Eridanus supervoid, and other supervoids like it are created by super massive black holes. These black holes would have to have the mass of many galaxies, I've even heard the phrase "universe-in-mass" being thrown about. I find this theory even harder to swallow, but I can't say that I wouldn't be excited to find out it's actually true.
It is possible that the supervoid is actually a massive black hole.