An enormous spaceborne lifeform that Voyager takes for a nebula rich in omicron particles in 2371, injuring it on the way in before returning to repair the damage.
The nebula creature is a single living organism thousands of kilometres across, which from outside looks like an ordinary nebula. It has something like a circulatory system and natural defences that react to intruders.
It appears in the Voyager episode The Cloud. Short of power, Janeway takes the ship in to collect omicron particles. Voyager has to punch through an energy barrier and is then pelted by an unknown compound that clings to the hull, forcing a retreat with less energy than it began with.
B'Elanna Torres and the Doctor find that the compound is organic and that the barrier was living tissue. Realising they have wounded a living being, the crew goes back inside and uses a nucleonic beam to help the wound close.