A dark matter nebula is 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.
As a guest aboard the Vulcan starship Yarahla, Jonathan Archer helped set up a graviton telescope while studying a dark matter nebula. (ENT: "Breaking the Ice")
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")
In 2371, the USS Voyager encountered trianic-based energy beings known as the Komar in a dark matter nebula in the Delta Quadrant. (VOY: "Cathexis")
Neelix said that he once lost a warp nacelle on a ship he was piloting through a dark matter nebula. (VOY: "Threshold")
An uncharted dark matter nebula was located in Cardassian space, near a Dominion ketracel-white 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")
According to Star Trek: Star Charts (p. 46) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the uncharted dark matter nebula was located in the Alpha Quadrant.