This is another post mostly summarizing some things that I’ve already mentioned in my earlier dark matter posts. While dark matter remains quite mysterious, the various pieces of evidence do give us some information about its properties, regardless of what it turns out being. A few of this are:
- Dark matter is “dark.” That is, it does not absorb or emit much light. This is the whole point of dark matter: it provides energy/mass density but has no direct affect on luminosity. Most of the time, this means that dark matter is made of something that fundamentally does not interact with the electromagnetic field at tree level (for Standard Model-like interactions) or at least interacts much more weakly than the usual Standard Model forces.
- Dark matter is long-lived. It existed in the early universe and exists today, and not much decaying (to Standard Model particles at least) can happen in the meantime if our cosmological models are to work. If it does decay, maybe it could decay to other types of dark matter.
- Dark matter is mostly “cold.” Cosmological measurements require that dark matter mostly be nonrelativistic so that the universe evolves in the correct way.
- Related to (3), dark matter has mass. Otherwise it couldn’t be nonrelativistic.
- Dark matter doesn’t interact much with Standard Model particles. Astronomical phenomenon like the Bullet Cluster show what appears to be dark matter flying right through large clouds of regular matter without doing much.
- Dark matter doesn’t interact with itself very much. Same as above. Plus, dark matter seems to exist mostly as large diffuse halos, so interactions certainly aren’t attractive enough to get the same kinds of small-scale features that we see in normal matter (stars, planets, rocks, etc).
In the ΛCDM model of cosmology, cold non-baryonic dark matter has to be produced in copious amounts in the early universe and the resulting evolution of its density must leave a relic density consistent with what is measured today. This results in some additional constraints if we want a dark matter model consistent with cosmology:
- A complete (for the purposes of adding dark matter) extended Standard Model must contain some mechanism for the creation of a huge amount of dark matter in the early universe
- This mechanism must not lead to large interactions with Standard Model particles that would violate the above points
If dark matter is assumed to be a new particle, then ideally it will be found in models that fix problems in the Standard Model. Having an elegant model explaining everything would be much more attractive than being forced to clumsily add particles in an ad hoc manner. From an experimentalist’s point of view, an effective model of dark matter interactions that is as generic as possible is probably best so that searches don’t tie themselves too much to highly constrained or contrived models. Experiments can search for generic dark matter candidates, and experimental results can be used to test the various models invented by theorists.