Colliding Clusters of Galaxies: Galaxy cluster 1E 0657-556, called the "Bullet Cluster," is actually two giant groups of galaxies that collided head-on. This composite picture was created from images taken by different space telescopes in X-ray and visible light. Astronomers think the collision made the dark matter around the galaxies visible, indicated by the blue regions, which bend the light from more distant galaxies in the background. The pink regions are hot gas stripped away in the collision. Dark matter is a still unidentified substance that makes up about 23 percent of the universe. It is thought to surround most galaxies, affecting their shapes by its gravity.
