It could be exotic.
This is what you can say about dark matter. It has mass. We know what the mass is. We know where a lot of it is. We know that it’s not luminescent and that it doesn’t reflect enough energy to be observed.
When you use the term dark matter, you’re not talking about a singular ‘substance’. In fact, when you talk about non-baryonic dark matter, you’re not talking about substance at all. Dark matter is not one thing, it’s a generic term for stuff you can’t observe. But what that various stuff is, is not known, again, because we aren’t able to observe it.
For example, when you’re talking about dark matter associated with galactic gravitation, you can talk about MACHOs, which can account for about 20% of all dark matter. That would be stuff like black holes, dwarf stars, neutron stars, and random dark bodies. This stuff is Baryonic matter, which is composed of particles that can form atoms. MACHOs are baryonic dark matter.
There is also non-baryonic matter - particles that don’t form atoms. 80% of dark matter is non-baryonic. Most theories that describe non-baryonic dark matter aren’t exotic, with one notable exception: Cold Dark Matter theory uses WIMPs, which are exotic and consistent with supersymmetry - also exotic. CDM competes with Hot Dark Matter, which has neutrinos as it’s associated particle - not exotic. HDM, Warm Dark Matter, and CDM describe non-baryonic matter. All of these have problems. HDM doesn’t work with cosmological structures, and CDM needs an exotic particle which might not even support CMD even when/if it is detected. CDM is an attractive and popular theory, but it’s association with WIMPs isn’t looking good.
Dark matter isn’t discrete - it’s not one thing. Baryonic dark matter is not exotic, and of the theories of non-baryonic dark matter, only CDM uses an exotic particle, and that particle has not been detected and remains theoretical. But it also has other problems, chiefly that it can’t account for all non-baryonic dark matter. So, like MACHOs, it also can’t account for the missing mass. But more specific to the relationship between WIMPs and CMD : in all the data collected by trillions of attempts to detect WIMPs, no dark matter signal has ever been detected. So, while WIMPs may eventually be detected, the hoped for relationship between WIMPs and CMD might never materialize - that’s not looking good.
