Dark matter is the invisible form of matter inferred from galaxy rotation and gravitational lensing.
Astronomers cannot see dark matter directly because it does not appear to emit, absorb, or reflect light in the ordinary way. Its presence is inferred from gravity: stars in many galaxies orbit faster than visible matter alone can explain, and massive galaxy clusters bend background light more strongly than their luminous contents predict.
The idea developed from early twentieth-century studies of galaxy clusters and became especially influential after observations of spiral-galaxy rotation curves. In the standard cosmological model, dark matter helps provide the gravitational scaffolding on which galaxies and larger structures form.
Dark matter is different from dark energy. Dark matter attracts through gravity and helps bind structures, while dark energy is the name given to whatever drives the universe’s accelerated expansion. Its exact particle identity remains unknown.