A dark matter nebula was a gaseous accumulation of material in space identical in its structure to a baryonic matter nebula caused by uncharacteristically high concentrations of dark matter. They could be extremely difficult to detect using conventional sensor technology.
In 2153, Archer and Sub-Commander T'Pol became the first to directly observe a dark matter nebula, illuminating it using metreon particles. T'Pol suggested naming it the "Robinson Nebula," in honor of Starfleet Captain A.G. Robinson, an old friend of Archer's who had recently died in a climbing accident. (ENT: "First Flight")
In 2367, the USS Enterprise-D used several modified photon torpedoes to illuminate the Mar Oscura Nebula, a previously-unexplored dark matter nebula. While exploring the nebula, the Enterprise encountered several spatial deformations, which made navigation extremely hazardous. (TNG: "In Theory")
An uncharted dark matter nebula was located in Cardassian space, near a Dominion ketracel-white storage facility destroyed by Captain Benjamin Sisko and his crew in 2374. Sisko ordered his damaged ship into the nebula to escape several pursuing Jem'Hadar fighters, whereupon they crashed on an uncharted planet. (DS9: "Rocks and Shoals")