English: This Hubble Space Telescope image shows 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, 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 equal as much as 3 million billion stars like our Sun (about 3,000 times more massive than 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 background galaxies like a funhouse mirror. This allowed them to come up with a mass estimate for the cluster.

The cluster was nicknamed El Gordo (Spanish for "the fat one") in 2012 when X-ray observations and kinematic studies first suggested it was unusually massive for the time in the early universe when it existed. The Hubble data have confirmed that the cluster is undergoing a violent merger between two smaller clusters.

For more information, visit: hubblesite.org/news_release/news/2014-22

Credit: NASA, ESA, and J. Jee (University of California, Davis)