This state of matter is usually called a plasma rather than a gas, but astronomy and astrophysics are very conservative disciplines that tend to preserve traditional terms. (If you want an extreme example of this look up "galactic clusters". Now that's a confusing name!) When it was first discovered in rich clusters of galaxies this plasma was called "cluster gas" or "intracluster gas" and the name has stuck. This adherence to tradition has served astronomers well because language doesn't change capriciously over time, and early literature is easily accessible to modern readers without the necessity of translation. We've known of the existence of this particular form of dark matter for many years. This paper purports to demonstrate that the cluster gas is more massive than the visible galaxies themselves.