A composite image of the Bullet Cluster, a much-studied pair of galaxy clusters that have collided head on. One has passed through the other, like a bullet traveling through an apple, and is thought to show clear signs of dark matter (blue) separated from hot gases (pink). Credit: X-ray: NASA/ CXC/ CfA/ M.Markevitch, Optical and lensing map: NASA/STScI, Magellan/ U.Arizona/ D.Clowe, Lensing map: ESO/WFI
