The Virgo Cluster: Hot Plasma and Dark Matter . ... The large area of X-ray emission, just below and left of center, is about 1 million light-years across. ... By adding up the amount of X-ray emitting gas astronomers have found that its total mass is up to 5 times the total mass of the cluster galaxies themselves - yet all this matter still does not produce nearly enough gravity to keep the cluster from flying apart! ... NASA Web Site Statements, Warnings, and Disclaimers . ...