Dark Matter
An invisible form of matter making up about 27% of the universe. Detected only through its gravitational effects on galaxies and large-scale structure.
Dark matter is a hypothesised form of matter that does not emit, absorb, or reflect electromagnetic radiation, making it invisible to telescopes. Its existence is inferred from numerous gravitational effects: galaxies rotate too fast for their visible mass, gravitational lensing reveals more mass than we can see, and the cosmic microwave background's structure requires dark matter to match observations.
Estimates suggest dark matter constitutes about 27% of the universe's total mass-energy, while ordinary atomic matter accounts for only ~5%. The remaining ~68% is dark energy, a separate phenomenon driving cosmic acceleration.
Despite decades of searching with underground detectors, particle accelerators, and astronomical surveys, no dark matter particle has been directly detected. Leading candidates include weakly-interacting massive particles (WIMPs), axions, and sterile neutrinos.