This galaxy cluster is shown as it existed when the universe was just 5 billion years old. The cluster is as massive as 300 trillion suns and is the most massive known cluster of its epoch. This image, which was taken between May and June 2002 with Hubble's Advanced Camera for Surveys Wide Field Camera, shows just the core of the cluster. Only about 50 galaxies are shown, but the cluster likely contains thousands. Dominating the core are a pair of large, reddish, elliptical galaxies (near the center of the image). Their red color indicates they hold a population of stars that are at least a billion years old. The red galaxies surrounding the central pair are also cluster members. Many of the other galaxies, including several blue galaxies, lie in the foreground.