I'm trying to understand the mechanisms of the anisotropies in the CMB. The general idea is that there are fluctuations in some field (e.g. inflation) and the baryonic and dark matter rush into the space compressing the fluid. The photon energy pushes back on just the baryonic matter while the dark matter pools in these wells. What I don't understand is why does the dark matter pool? I assume it gathers some momentum on it's way into the gravity well, so why isn't that momentum conserved? The dark matter has no way of losing energy; it will just transfer it from potential to kinetic and back again, so why doesn't the dark matter oscillate around the fluctuations always returning to it's starting potential?