This is a small vermiform creature encountered by Captain Kathryn Janeway and crewmembers Harren, Tal and Telfer on a mission to explore the vicinity of a class T gas giant located within a nebula filled with spatial anomalies, in the Delta Quadrant. After their shuttlecraft was damaged by one such anomaly (or possibly a dark matter body collision as the scans suggest), Telfer was briefly abducted by unknown means. Scan analysis suggested he wasn't anywhere in space or subspace as far as the sensors got, indicating that he may have been taken to a different dimension. Telfer was returned shortly afterwards with the strange worm-like lifeform inside his body, where it attempted to communicate with him via his nervous system.
Tricorder analysis and attempts to use the transport to remove the parasite both have no effect on the creature. Under the alien's control, Telfer is able to walk through a force field, and is stunned by a phaser shot from Janeway, leading the creature to leave his body and attempt to enter the shuttle's computer systems instead, tapping with the environmental controls. Convinced that the creature is hostile, Harren disintegrated it with his phaser, leading to the appearance of numerous alien ships pursuing the shuttle. Telfer later revealed the only thing he could grasp from the alien's attempt to communicate was the phrase "do not belong", which could be interpreted in many ways. Fortunately for the shuttle crew, they were able to escape the pursuit and make it back to Voyager.
The origins of the alien creatures are never revealed. Although unconfirmed, Janeway has speculated that the creatures are made of some form of dark matter, but Harren remained skeptical of the idea, claiming that complex molecules of that nature would collapse under their own weight.