The Robinson Nebula.
A dark matter nebula is a gaseous accumulation of material in space which can be extremely difficult to detect using conventional sensor technology.
As a guest aboard the Vulcan ship Yarahla, Jonathan Archer helped set up a graviton telescope while studying a dark matter nebula. (ENT: "Breaking the Ice")
In 2153, Archer and Subcommander 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")
The Mar Oscura Nebula
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 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")
A dark matter nebula in Cardassian space.
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")