Extragalactic dark matter and direct detection experiments

Abstract

Recent astronomical data strongly suggest that a significant part of the dark matter, composing the Local Group and Virgo Supercluster, is not incorporated into the galaxy haloes and forms diffuse components of these galaxy clusters. Apparently, a portion of the particles from these components may penetrate into the Milky Way and make an extragalactic contribution to the total dark matter containment of our Galaxy. We find that the particles of the diffuse component of the Local Group are apt to contribute $\sim 12%$ to the total dark matter density near the Earth. The particles of the extragalactic dark matter stand out because of their high speed ($\sim 600$ {km/s}), i.e. they are much faster than the galactic dark matter. In addition, their speed distribution is very narrow ($\sim 20$ {km/s}). The particles have isotropic velocity distribution (perhaps, in contrast to the galactic dark matter). The extragalactic dark matter should give a significant contribution to the direct detection signal. If the detector is sensitive only to the fast particles ($v<450$ {km/s}), the signal may even dominate. The density of other possible types of the extragalactic dark matter (for instance, of the diffuse component of the Virgo Supercluster) should be relatively small and comparable with the average dark matter density of the Universe. However, these particles can generate anomaly high energy collisions in direct dark matter detectors.