Observational evidence for self-interacting cold dark matter

Cosmological models with cold dark matter composed of weakly interacting pa rticles predict overly dense cores in the centers of galaxies and clusters and an overly large number of halos within the Local Group compared to actu al observations. We propose that the conflict can be resolved if the cold d ark matter particles are self-interacting with a large scattering cross sec tion but negligible annihilation or dissipation. In this scenario, astronom ical observations may enable us to study dark matter properties that are in accessible in the laboratory.