Messier 87 is a supergiant elliptical galaxy in the constellation Virgo. The image on the left (courtesty of UCO/Lick Observatory) shows M87 in visible light. The image on the right (courtesy of the Smithsonian Astrophysical Observatory) is an x-ray image from the Einstein Observatory showing the region around M87. Most of the x-rays come from a halo of hot gas enveloping M87 and nearby galaxies. The gas cloud should have dissipated into space, but it seems to be held in place by gravity from a large amount of undetectable mass around M87—strong evidence for the existence of dark matter.