Ohio State Univ, Dept Phys, Columbus, OH 43210 USA
; CALTECH, Pasadena, CA 91225 USA
; Chinese Acad Sci, Inst High Energy Phys, Beijing 100039, Peoples R China
After neutralinos cease annihilating in the early Universe, they may still scatter elastically from other particles in the primordial plasma. At some point in time, however, they will eventually stop scattering. We calculate the cross sections for neutralino elastic scattering from standard-model particles to determine the time at which this kinetic decoupling occurs. We show that kinetic decoupling occurs above a temperature T similar to MeV. Thereafter, neutralinos act as collisionless cold dark matter.