Non-luminous matter that sensors struggle to detect, responsible in Star Trek for hazards including the hull-warping pockets of the Mar Oscura Nebula and the asteroid mined by Discovery.
Dark matter is matter that gives off no light and interacts only weakly with ordinary matter. In Star Trek it tends to appear as a navigational hazard or as an exotic resource.
In The Next Generation episode In Theory, the Enterprise-D explores the Mar Oscura Nebula, where pockets of dark matter cause parts of the ship to fade briefly out of phase, killing a crew member, and Picard pilots a shuttle ahead to find a safe path. Voyager's Cathexis is set by a dark matter nebula, and in One Small Step a dark matter asteroid strikes the graviton ellipse.
Enterprise's First Flight has Archer and T'Pol make a dark matter nebula visible by exciting it with spatial charges. In Discovery's second season the crew capture a piece of an asteroid made of dark matter in Brother, material that the crew bring aboard for study and later put to use in their research.