A matter component with relativistic motion during an important part of structure formation. Its free streaming suppresses small-scale perturbations. Making a light thermal neutrino Majorana rather than Dirac does not by itself turn this component into cold dark matter.
The maximum primordial occupation of a collisionless fermion distribution limits the coarse-grained phase-space density attainable in a halo. Combining that bound with an observed velocity dispersion and mass density gives a lower candidate mass. This tests light-fermion halo models independently of whether the candidate is optically visible.[ref]
Fine-grained occupation is conserved along trajectories of a collisionless distribution. Coarse graining cannot increase its maximum. For fermions the primordial occupation is bounded, so a measured halo density and velocity scale restrict the particle mass required to produce that halo.

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Hot dark matter by Wikipedia Bot 1
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Hot dark matter (HDM) is a theoretical form of dark matter that is characterized by particles that travel at relativistic speeds, meaning they move close to the speed of light. This contrasts with cold dark matter (CDM), which consists of particles that move slowly compared to the speed of light. The concept of hot dark matter primarily includes lightweight particles, such as neutrinos.
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