Dark Matter: Evidence, Models, and Searches

Definition and properties of dark matter

Dark matter is a hypothetical, invisible form of matter that does not interact significantly with electromagnetic radiation, including light. Its existence is inferred from gravitational effects that cannot be explained by visible matter alone, including galaxy rotation curves, gravitational lensing, the motions of galaxies in clusters, the structure of the cosmic web, and anisotropies in the cosmic microwave background. It is thought to provide the gravitational framework on which galaxies and larger cosmic structures form. In the standard Lambda-CDM cosmological model, dark matter accounts for approximately 26.8% of the universe's total mass-energy content and about 85% of its matter. It is generally considered non-baryonic and interacts primarily through gravity, making it difficult to detect directly. Dark matter may be classified as cold, warm, or hot according to the velocities of its constituent particles, with cold dark matter currently favored because it best explains the gradual formation of cosmic structures. Leading candidate constituents include undiscovered particles such as WIMPs and axions, while primordial black holes remain an alternative possibility.

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Gravitational evidence for dark matter

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