[chris (OP)]

Sarah wrote from Australia to ask:

I was reading recently about how dark matter does not collapse down from a cloud to a disc shape like normal matter does, because it cannot radiate heat away by emitting light/heat. Does this mean dark matter can never heat up or cool down?

Can anyone help?

[Kryptid]

If one defines temperature as the average velocity of particles in a substance, then a cloud of dark matter should have a temperature of some kind. It may not be able to absorb or release photons, but it could potentially change its particle velocity in other ways. Since dark matter interacts via gravity, a cloud of dark matter may be able to heat up if a spinning black hole were to pass through it (dark matter particles entering its ergosphere but not its event horizon could have their velocity boosted). If dark matter clouds can emit energy in the form of gravitational waves, they should cool down (very, very slowly) over time as well.

[yor_on]

it's a very strange form of matter, if it exist. Doesn't act with anything except gravity as far as I get it? If it is a gas consisting of 'particles' they don't interact at all, what does that do to the standard theory? It's a very weird idea to me.

[chris (OP)]

Since dark matter interacts via gravity, a cloud of dark matter may be able to heat up if a spinning black hole were to pass through it (dark matter particles entering its ergosphere but not its event horizon could have their velocity boosted).

Or, presumably, as it collapses together it can cause frictional heating, in the same way that the Earth heated up as it formed?

[evan_au]

The properties of the hypothetical Dark Matter are still somewhat unclear. The most common theory ("Cold Dark Matter") suggests that Dark Matter:

• Does interact with "Normal" matter via gravity
• Does not interact with "Normal" matter by the electromagnetic force (including light & heat) or the Strong Nuclear Force
  
• Is subject to expansion of the universe, so the Dark Matter near us has roughly the same velocity as our galaxy (ie it is fairly "Cold"); the relative velocity is low enough that the Dark Matter center of mass is orbiting the galactic center of mass, forming a Dark Matter Halo.

If Dark Matter were "Hot" (eg relic neutrinos from the Big Bang), it would have a very high velocity relative to our galaxy, would not be trapped by the galactic gravitational field, and would not form a galactic halo.
 
At this point in time, it is not clear if Dark Matter:

• Interacts with itself - but some teams are searching for signs of Dark Matter/anti-Dark Matter annihilation in X-Rays and Gamma rays
• Interacts via the Weak Nuclear Force; this might be hard to detect if the energy and cross-section of the interaction is low
• Interacts with itself using some new force (stronger than gravity) which could carry away energy, allowing Dark Matter to cool down and clump together like "Normal" matter. But the average velocity relative to our galaxy would remain the same as if teh Dark Matter remained as individual particles.

See: https://en.wikipedia.org/wiki/Dark_matter#Cold_dark_matter

Since there is thought to be far more "Dark Matter" than "Normal Matter", it is arguable which kind is really normal...

[sceptic-eng]

Dark matter force is a magnetic force and is responsible for the supermassive black hole at the centre of all galaxies around which all the stars spin.  To heat up or cool down is an 3D energy equation not just dark magnetic spin force effect