A galaxy cluster is the largest gravitationally bound structure in the universe, holding hundreds to thousands of galaxies. Most of its ordinary matter is not inside those galaxies but spread between them as a plasma heated to tens of millions of degrees and glowing in X-rays. Dark matter dominates the total mass, typically outweighing everything visible by a factor of five or more. Clusters bend the light of objects behind them into gravitational lenses, and collisions between them, such as the Bullet Cluster, supply some of the strongest evidence that dark matter is real.