Dark matter is matter inferred from its gravitational effects that does not emit, absorb, or scatter enough electromagnetic radiation to account for those effects as ordinary baryonic matter.
Cold dark matter is nonrelativistic early enough that its primordial free-streaming length is small compared with the cosmological structures of interest.
Warm dark matter has a primordial collisionless free streaming length large enough to suppress the formation of low-mass haloes, while remaining nonrelativistic early enough to cluster on galactic and larger scales.
Articles by others on the same topic
Dark matter is a form of matter that does not emit, absorb, or reflect light, making it invisible and detectable only through its gravitational effects on visible matter. It is believed to make up about 27% of the universe's mass-energy content, while ordinary matter (like stars and planets) constitutes only about 5%. The remaining 68% is attributed to dark energy, which is thought to be responsible for the accelerated expansion of the universe.