One of the Recently Discovered Components of Clusters of Galaxies

One of the recently discovered components of clusters of galaxies that may have some bearing on the dark matter problem is:

A) large numbers of cool, dark, brown dwarf stars, previously unknown, in the galaxies.
B) X-ray measurements of substantial amounts of very hot intergalactic gas in clusters.
C) a tremendous flux of neutrinos, which have recently been shown to have mass, coming from these clusters.
D) large numbers of small but massive black holes within every galaxy.