Dark matter

Dark matter is matter inferred from gravitational effects on galaxies and cosmic structure but not directly observed through emitted, absorbed, or reflected light. Its physical nature remains unknown.

Galaxy rotation curve: A graph of orbital speed versus distance from a galaxy’s center. Many galaxies rotate too quickly at their outskirts for visible matter alone to explain.
General relativity: Einstein’s theory of gravity, describing it as the curvature of spacetime by energy and matter. It predicts how unseen mass bends light and affects cosmic motion.
Coma Cluster: A rich galaxy cluster whose member motions indicate far more mass than its visible galaxies provide. Fritz Zwicky’s analysis of its galaxy speeds supplied an early mass discrepancy.
Modified Newtonian dynamics: A theory that modifies Newtonian gravity at very low accelerations to explain galaxy dynamics without dark matter. It can fit some galactic rotation patterns but faces challenges across cosmological evidence.
Weakly interacting massive particle: A hypothetical massive particle that interacts weakly with ordinary matter and could account for dark matter. Direct searches have not yet detected the expected interactions.
Gravitational lensing: The bending and distortion of light by gravity along its path. Lens distortions map mass, including dark matter, without requiring it to emit light.
Virial theorem: A relation between the average kinetic and potential energies of a stable gravitational system. It lets astronomers estimate cluster masses from galaxy motions.
Bullet Cluster: A pair of colliding galaxy clusters whose hot gas and gravitationally mapped mass are spatially separated. It offers a direct comparison between ordinary matter and the location of most gravitating mass.
Neutrino: A nearly massless elementary particle that interacts through gravity and the weak nuclear force. Known neutrinos contribute too little, and move too quickly, to account for most dark matter.
Axion: A hypothetical light particle proposed to solve the strong CP problem and considered a dark matter candidate. Experiments search for axions through their possible conversion into photons in magnetic fields.

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