A composite image of the Bullet Cluster. The Bullet Cluster is a much-studied pair of galaxy clusters, which have collided head on. One has passed through the other, like a bullet traveling through an apple. In the Bullet Cluster, this is happening across our line of sight, so we can clearly see the two clusters. Credits: NASA/CXC/CfA, M.Markevitch/STScI, Magellan/U.Arizona, D.Clowe/ESO/WFI
