Dark matter

Astronomers infer dark matter because the gravitational effects observed in galaxies and galaxy clusters exceed what their visible contents alone would predict. The motions of stars and gas in galaxies, the dynamics of galaxy clusters and the bending of light from distant objects by gravitational lensing provide complementary ways to trace the distribution of mass. The Bullet Cluster, formed by colliding galaxy clusters, is a notable case: maps inferred from lensing place much of the mass apart from the hot gas, the dominant ordinary-matter component visible in X-rays. Such observations support the presence of unseen mass, though they do not directly identify what it is.

Measurements of the cosmic microwave background—the relic radiation from the early universe—also constrain the amounts and properties of cosmic components. In the standard cosmological model, cold dark matter helps explain how cosmic structure developed. The Planck Collaboration's analysis of its 2018 observations reported a physical cold-dark-matter density parameter of Ωc h² = 0.120 ± 0.001 within its baseline ΛCDM model. This is a model-dependent parameter, not a direct inventory of individual dark-matter particles. Estimates that dark matter accounts for roughly 27% of the universe's total energy density refer to a cosmological composition inferred from combined observations and the standard model.

Dark matter is not known to be a single specific substance. Candidate explanations include weakly interacting massive particles, axions and, in some scenarios, primordial black holes. These are hypotheses with distinct predicted properties and possible experimental signatures. Experiments search for dark matter through several routes: direct detection of interactions in laboratory detectors, indirect searches for products of possible dark-matter processes, and accelerator searches for events with missing energy or momentum. No candidate has yet been established as the dark matter through confirmed detection.

Dark matter and dark energy are separate concepts. Dark matter is inferred to have mass and to contribute to gravitational attraction and the formation of structure. Dark energy is the name for the component associated with the observed accelerated expansion of the universe in the standard cosmological account. The shared word “dark” signals that their underlying nature is not understood, not that they are the same phenomenon.

The gravitational evidence for additional mass is strong across multiple settings, but the identity and microscopic properties of dark matter remain open questions. The conclusions depend on interpreting observations within gravitational and cosmological models; measurements of gravitational effects do not by themselves constitute detection of a particular particle. Continued astronomical observations and laboratory experiments test competing explanations.