Discuss the evidence supporting the existence of dark matter in the universe and the leading theories about its composition.

Evidence for dark matter includes the rotation curves of galaxies, gravitational lensing, and the cosmic microwave background radiation, all suggesting more mass exists than can be accounted for by observable matter. Leading theories about its composition include Weakly Interacting Massive Particles (WIMPs), axions, and sterile neutrinos, which are particles that interact through gravity but not through electromagnetic forces, making them difficult to detect directly.