1. Non-Baryonic
Results of the CMB, together with the predictions from Big Bang nucleosynthesis, suggest that only 4-5% of the total energy is made out of ordinary, baryonic matter. Hence, we must conclude that DM is non-baryonic.
Neutrinos - A special mention because they are the only viable non-baryonic DM candidate with a place in the Standard Model. Neutrinos are very abundant particles in the universe with a (very small) mass. They also interact feebly with ordinary matter (only through electro-weak interactions). Hence, they are in fact, a component of DM. But,
- Neutrinos are too light.
- Neutrinos are relativistic (hot) at the epoch of structure formation.
2. Neutral
DM particles must be electrically neutral. Otherwise, they would scatter light and thus not be “dark”.
3. Non-relativistic
At the epoch of structure formation, DM must be non-relativistic (cold) since relativistic (hot) DM has a larger free-streaming length (average distance travelled by DM before falling into a potential well) that leads to inconsistencies with observations.
However, at the Galactic scale, cold dark matter simulations lead to the occurrence of too much substructure in dark matter haloes. If dark matter was warm (mass of 2-3 keV), this problem would be alleviated.
4. Long-lived
Clearly, DM must be long-lived (stable) since its footprint can be observed in the CMB anisotropies, its presence is essential for structure formation and we can feel its gravitational effects in clusters of galaxies and galaxies today.
Stable DM candidates are common in models where new discrete symmetries are imposed by ensuring DM particles are the lightest with an exotic charge (decay is forbidden). Eg. Supersymmetry (R-parity is imposed).
However, stability is not required by observation. DM particles can decay, as long their lifetime is longer than the age of the universe. They feature very small couplings.
5. Collision-less
Dynamical systems like cluster collisions set an upper bound to self-interactions of DM particles. Observations suggest that the DM components in these objects are mostly collision-less, thus behaving very differently from ordinary matter.