system
Given the following post, respond as if you're commenting on Reddit:
human
If dark matter is made of particles that only interact through the weak force and gravity, are these particles an excitation of an field? 

Since particles are excitations of different fields (Gluons, excitations of the strong gauge field. Photons, W bosons, and Z bosons, excitations of the electroweak gauge fields.), has dark matter an associated field?
gpt
Absolutely! As far as we know *all* particles are excitations of some quantum field. There are plenty of candidates for the dark matter which fall in this standard description.

So maybe you're wondering how these particles are excited in the first place, if they only interact weakly (possibly *only* through gravity)? And that's a very good question, and one we don't know the answer to. It depends on the particular dark matter candidate.

A particularly popular mechanism is *thermal production*. In the very early Universe, when energies were very high, dark matter decays into other particles (assuming they exist) were balanced out by the opposite reaction, in which dark matter particles were *produced* from other particles. This kept dark matter in equilibrum, with an abundance which we can compute from Boltzmann's equation. Once the temperature of the Universe dropped below the point where these reactions were energetically favorable, dark matter production ceased and the dark matter abundance "froze out," meaning the number of dark matter particles stayed more or less constant (up to the occasional decay).

People like this scenario because it turns out that if you calculate the interaction cross section which gives the correct dark matter density today, assuming they were thermally produced, it turns out to be *exactly* the same as the cross section for a WIMP (weakly interacting massive particle) with some typical mass.

Most dark matter candidates are produced this way. There are a variety of other alternatives out there, for example one in which dark matter particles - like normal matter - had an initially asymmetry between matter and anti-matter, so that the two didn't all annihilate leaving nothing behind. In that case, the dark matter might have a similar mechanism to normal matter. But we don't really understand why normal matter has this asymmetry either, so there's still a lot of speculation.