Dark matter evidence. Composite image showing the cluster of galaxies NGC 2300 (seen at optical wavelengths) and the recently discovered cloud of gas (magenta, in X-ray emission) in which the galaxies are embedded. The image of the gas cloud was taken by the ROSAT X-ray satellite and is considered to be strong evidence for the presence of dark matter - dark because it does not emit detectable radiation. The cloud is formed by hot gas which has a temperature of about 10 million Kelvin. The gravitational pull of the cluster is not enough to hold such a cloud together. Some astronomers have suggested that it is dark matter that prevents the cloud from dispersing in space.