Dark matter lifeform
The "dark matter lifeforms" were a race of creatures dwelling outside normal space-time and subspace in an unknown parallel realm. 

In 2376, an away team from the Federation starship  encountered the alien lifeform during an exploration of a class T cluster in the Delta Quadrant. The warp drive of the  was disabled by a wave of spatial fluctuation and one of the team's members, Crewman William Telfer, was transported away to the alien realm and returned with the lifeform in his body. It took control of his nervous system and established rudimentary telepathic contact with him. Attempts to scan, or lock onto it and beam it out of him failed. 

The leader of the team, Captain Kathryn Janeway, surmised that it could be a dark matter lifeform. However, Federation theoretical physics of the 24th century seemed to preclude the existence of such a lifeform, as molecules so complex would collapse under their own weight. The lifeform subsequently left Telfer's body and accessed the systems of the Delta Flyer, causing Crewman Mortimer Harren to panic and disintegrate it with a phaser.

Shortly after, the lifeforms advanced on the Flyer, presumably seeking retribution. Janeway was able to disable their spatial fluctuation wave by triggering a chain reaction in the radiogenic rings of a T-class gas giant. 

It was never conclusively proven if these lifeforms were, in fact, dark matter lifeforms, nor was it proven if such lifeforms could exist. ()
de:Dunkle-Materie-Lebensform
Unnamed species
Lifeform classifications