Number 259 (Story #2), February 21, 1996 by Phillip F. Schewe and Ben Stein DARK MATTER, LIKE LUMINOUS MATTER, IS HIERARCHICAL. That is, it congregates at the galactic level and at the level of galaxy clusters. This view is based on new observations made with the orbiting Japanese x-ray telescope ASCA, which recorded x-ray emissions from gas in the Fornax galaxy cluster (Y. Ikebe et al., Nature, 1 February 1996). The density of dark matter at any location is deduced from the density of the hot (up to 10**8 K) gas, which is probably held in equilibrium by the gravitational influence of the unseen dark matter thereabouts. The ASCA scientists suggest that one explanation of their measurements is the presence of two different kinds of dark matter. This is in keeping with some hybrid cosmological models which propose that cold dark matter (e.g., axions) influences affairs at the galactic level and hot dark matter (e.g., massive neutrinos) at the cluster level. (Science News, 10 Feb. 1996.)