Around 760 million light years from Earth lies the galaxy cluster Abell 85, a mighty collection of about 500 galaxies all orbiting one another. At its center is the enormous elliptical galaxy Holmberg 15A… and I do mean enormous; some estimates put it at well over 50 trillion times the mass of the Sun, dozens of times more massive than the Milky Way. All big galaxies have a supermassive black hole in their hearts, millions or billions of times the Sun's mass. In very rough terms the black hole size scales with the galaxy, and since the galaxy is so huge it's thought the black hole at its core may be as well. It's never been measured directly, but using various indirect bellwethers its mass has been estimated at tens of billions to well over a hundred billion times the Sun's mass. If true, it's the most massive black hole known. However, its actual size is in fact unknown.