I understand there are no friction forces to slow it down but there are other means of dissipating kinetic energy. Every time 2 dark matter particles come close enough to interact gravitationally there should be some exchange of momentum. If there is a cloud of dark matter particles, such as in and around a galaxy, there would be constant re-distribution of momentum. Each time a particle acquires enough momentum to escape the cloud the average momentum of the particles in the cloud is reduced. This would be a sort of evaporation that continually lowers the average temperature of the cloud. Cosmic expansion would create a difference between the amount of energy gained from incoming particles and that lost with outgoing particles. The result of all this should be small, dense, bodies of dark matter. I understand that small, dense, non-luminous bodies have been ruled out as dark matter candidates. What prevents dark matter from behaving as I have suggested?