Mark Whittaker says the dark matter cant cool

[BkBkBk]

I'm just watching a lecture series with Mark Whittaker in and he states (while talking about primordial roughness and sachs-wolfe effect) "the property that atomic matter has dark matter does not, is that it can cool" but he leaves it at that and provides no further clarification,and I am struggling to find anything in my searches. If it is true, how has he come to this conclusion, is it intrinsic to the candidates for dark matter,or something we have a good theoretical underpinning for? Surely its not something we have actually measured,since we are yet to find dark matter, could somebody please clarify?

thanks

 

[Bob S]

All stable nuclear and atomic particles have a large Coulomb or atomic scattering cross sections for transferring momentum and energy from one particle to another. We are unable to detect weakly-interacting dark matter, in part because its scattering cross section with normal matter is negligibly small. This probably is also true for the dark-dark scattering cross section, leading to the predicted persistent galactic halos.

Bob S

 

[LukeD]

Normal matter comes to thermal equilibrium through collisions and exchange of radiation, both of which happen mostly through the electromagnetic force.
Dark matter does not interact through the electromagnetic force, so the usual way of achieving thermal equilibrium is gone.