Dark matter is an invisible, hypothetical form of matter that doesn't interact with light, yet its presence is inferred through gravitational effects like galaxy rotation, gravitational lensing, and cosmic microwave background anisotropies. In the standard Lambda-CDM model, it makes up about 26.8% of the universe's mass-energy content (and 85% of its total mass), acting as the gravitational scaffolding for cosmic structures like galaxies and the cosmic web. Scientists believe it could be composed of undiscovered particles like WIMPs or axions, or possibly primordial black holes, but it remains extremely difficult to detect because it only interacts via gravity. While the astrophysics community largely accepts its existence, a minority propose modified gravity theories like MOND, though none can fully explain all observations. The concept has a long history, with early hypotheses from Lord Kelvin in 1884, Henri Poincaré in 1906, and Jan Oort in 1932, who studied stellar velocities to suggest unseen mass. This makes dark matter one of the most profound mysteries in modern cosmology, shaping the universe's large-scale structure since the Big Bang.