A supposed dark matter lifeform
A dark matter lifeform was a theoretical a lifeform composed of dark matter, dwelling outside normal space-time.
In 2376, an away team from the Federation starship USS Voyager, encountered an alien lifeform during an exploration of a class T cluster in the Delta Quadrant. One of the team's members, Crewman William Telfer, was transported away and then was returned with the lifeform in his body. It took control of his nervous system and established rudimentary telepathic contact with him. Attempts to lock onto it and beam it out of him failed.
The leader of the team, Captain Kathryn Janeway, Voyager's master and commander, surmised that it was 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 Flyer's systems, causing Crewman Mortimer Harren to panic and destroy it with a phaser.
Shortly after, more of these lifeforms advanced on the Delta Flyer seeking retribution. Captain Kathryn Janeway was able to disable them 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 actually existed.(VOY: "Good Shepherd")