darkmatter

Dark matter is a form of matter that does not emit, absorb, or reflect enough electromagnetic radiation to be detected directly. Its presence is inferred from gravitational effects on visible matter, radiation, and the large-scale structure of the universe. In the standard cosmological model, dark matter makes up about 27% of the universe's energy density, with ordinary matter around 5% and dark energy about 68%.

Evidence for dark matter includes galaxy rotation curves that remain flat at large radii, velocity dispersions in galaxy clusters, gravitational lensing by dark-matter halos, and the cosmic microwave background anisotropies. The Bullet Cluster is often cited because its gravitational potential does not coincide with the luminous gas, suggesting a collisionless component consistent with dark matter.

Most candidates are non-baryonic particles that interact weakly with ordinary matter. The leading class is cold dark matter, with particles such as WIMPs or axions. Other candidates include sterile neutrinos and other beyond-Standard-Model particles. While these particles are hypothetical, they fit observations of structure formation; alternative theories like Modified Newtonian Dynamics offer different explanations for some phenomena but do not reproduce all data as consistently.

Detection efforts include direct searches for nuclear recoils, indirect searches for annihilation or decay products, and collider production experiments. To date no conclusive detection has been made, and experiments have set limits on interaction cross sections and masses. Ongoing and planned experiments aim to probe broader parameter space and refine our understanding of dark matter.