Dark matter is inferred from galaxy rotation curves, velocity dispersions, gravitational lensing, structure formation, and the CMB. A successful model must be cold or sufficiently nonrelativistic early enough to seed hierarchical clustering and match observed power spectra.

For a weakly interacting massive particle, the relic abundance is set by freeze-out when annihilations can no longer compete with expansion. A useful intuition is that the final density is inversely related to the thermally averaged annihilation rate $\langle\sigma v\rangle$.

Axion dark matter can arise from vacuum misalignment: the axion field initially sits away from the minimum of its potential and later begins oscillating when $H\sim m_a$. These oscillations behave like cold matter on large scales.

Direct detection, indirect detection, collider searches, and cosmological observables jointly restrict dark matter models. Small-scale structure, halo profiles, and CMB anisotropies can probe self-interactions, free-streaming, and energy injection.