This space wallpaper is a Hubble Space Telescope image of the most massive cluster of galaxies ever seen to exist when the universe was just half of its current age of 13.8 billion years. The cluster, catalogued as ACT-CL J0102-4915 and nicknamed El Gordo, contains several hundred galaxies swarming around under the collective gravitational pull. The total mass of the cluster, as refined in new Hubble measurements, is estimated to weigh as much as 3 million billion stars like our Sun (about 3,000 times as massive as our own Milky Way galaxy) — though most of the mass is hidden away as dark matter. The location of the dark matter is mapped out in the blue overlay. Because dark matter doesn't emit any radiation, Hubble astronomers instead precisely measure how its gravity warps the images of far background galaxies like a funhouse mirror. This allowed them to come up with a mass estimate for the cluster.
(Image: © NASA, ESA, and J. Jee (University of California, Davis))