In Star Trek, dark matter nebulae are accumulations of dark matter, that are difficult to detect on sensors unless illuminated by metreon particles.  In the episode "Cathexis",  the USS Voyager encountered trianic energy beings known as the Komar in a dark matter nebula in the Delta Quadrant.  In the Star Trek: Enterprise episode "First Flight", a dark matter nebula was observed by Captain Archer and Subcommander T'Pol and given the name the Robinson Nebula. In the Star Trek: The Next Generation episode "In Theory", the USS Enterprise-D explored a previously uncharted dark matter nebula - the Mar Oscura Nebula - which was littered with spatial deformations, making travel hazardous.  In the Star Trek Deep Space Nine episode "Rocks and Shoals", Captain Benjamin Sisko ordered his damaged ship into a dark matter nebula to escape several pursuing Jem'Hadar fighters, whereupon they crashed on an uncharted planet.

A dark matter nebula depicted in 'Rocks and Shoals'

Some real-life nebula may indeed contain large concentrations of dark matter. The famous "Snake" or "S" dark nebula in Ophiuchus, for example, is a concentration of obscuring dust and dark matter in space that is opaque and doesn't let the stars behind show through it. Dark matter is hypothetical matter that does not interact with the electromagnetic force, but whose presence can be inferred from gravitational effects on visible matter. According to present observations of structures larger than galaxies, dark matter and dark energy account for the vast majority of the mass in the observable universe.

Dark matter is also thought to play a central role in structure formation and galaxy evolution. It is estimated that nly about 4% of the total energy density in the universe (as inferred from gravitational effects) can be seen directly. About 22% is thought to be composed of dark matter. The remaining 74% is thought to consist of dark energy, an even stranger component, distributed diffusely in space. Determining the nature of this missing mass is one of the most important problems in modern cosmology and particle physics.

The term 'dark matter nebula', should not be confused with the real astronomical term 'dark nebula'.  See 'Black Cluster'.

Gravity pulls dark matter into clumps, perhaps forming 'dark galaxies'