What is dark matter?
Dark matter is the name given to matter inferred from its gravitational effects even though it does not interact with light as ordinary matter does. Its nature remains unknown; “dark” describes the lack of direct light emission or reflection.
Evidence for its existence
The rotation speeds of stars and gas in outer galaxies differ from expectations based only on visible matter. Motions within galaxy clusters and gravitational lensing, the bending of distant light by mass, also point to additional mass. Patterns in the cosmic microwave background and large-scale structure are studied together. No single measurement should be presented as absolute proof; independent observations and alternative models are compared. Dark matter is not automatically identical to black holes or ordinary obscuring dust.
What is known and unknown?
The identity of dark-matter particles has not been established; candidate particles and modified-gravity proposals are being tested. A successful candidate must agree with cosmological observations as well as laboratory or astrophysical measurements. Dark energy is a separate concept associated with changes in the universe’s expansion rate. The shared word “dark” does not make them the same thing. The strength of evidence for an unseen component must be distinguished from uncertainty about its microscopic identity.