nonbaryonic

Nonbaryonic refers to matter composed of particles that are not baryons. In cosmology, nonbaryonic matter denotes the dominant form of matter in the universe that is not made of protons, neutrons, and other baryons. It is distinct from ordinary (baryonic) matter such as stars, gas, and dust. The standard model of cosmology, ΛCDM, posits that nonbaryonic matter accounts for about 27% of the energy density of the universe, while baryonic matter makes up about 5%, and dark energy the remainder.

Nonbaryonic matter is characterized by its weak or negligible interactions with electromagnetic radiation, which makes it difficult to observe directly. Its existence is inferred from indirect evidence: the flat rotation curves of galaxies, gravitational lensing around galaxy clusters, measurements of the cosmic microwave background, and the suppression of small-scale structure relative to that expected from baryons alone.

Classification by thermal history distinguishes hot, warm, and cold dark matter. Hot dark matter consists of light, fast-moving particles that erase small-scale structure; cold dark matter is nonrelativistic and clumps on small scales; warm dark matter has intermediate velocities.

The leading candidates for nonbaryonic matter include weakly interacting massive particles (WIMPs), axions, and sterile neutrinos. Other possibilities exist, such as dark photons or other exotic particles. Researchers pursue direct detection (nuclear recoils in detectors), indirect detection (annihilation or decay products), and collider searches, but no conclusive detection has been made as of now. The concept remains central to explanations of cosmic structure and the evolution of the universe.