Composite image of the Perseus galaxy cluster. The image combines X-ray and optical light observations taken by ESA's XMM-Newton?檚 European Photon Imaging Camera (EPIC) and the Digitzed Sky Survey II, respectively. The diffuse coloured regions represent hot gas at different temperatures which changes with radius, with cooler regions towards the centre. The Perseus cluster contains thousands of galaxies and is one of the most massive objects in the known universe.