Space amoeba

The space amoeba was a large space dwelling cosmozoan.

Overview
The creature shared many qualities similar to a single celled organism but was a vast creature that lived in the vacuum of space. It was surrounded by a field of negative energy which was toxic to humanoid life. The amoeba was able also able to reproduce through the use of cell division. Its sheer size meant that starships could enter into its body where they were exposed to the negative energy field that killed any possible crew within it. However, it was possible to withstand exposure to this energy which was why the creature had cell anti-bodies present within it which attacked any intruders within the cell membrane.

The core of the amoeba was the cell nucleus which was vulnerable to damage. Any harm to it had the potential of killing the organism.

Primary universe
One such organism of unknown origin was encountered by the USS Enterprise in 2268 when it was in route to Starbase 6. The crew of the Starfleet vessel was diverted from their mission in order to investigate the destruction of an inhabiated world in the Gamma 7A system as well as the loss of the entire Vulcan crew of the USS Intrepid. After discovering the massive amoeba-like lifeform, the crew discovered the danger it posed especially of its reproductive ability allowing it to overrun the galaxy and destroy whole planets. Due to this threat, the crew of the Enterprise destroyed the creature by detoating an antimatter charge within it. 

Mirror universe
In the Mirror universe, a similar organism was ravaging the galaxy in 2293 but instead to move into the territory of the Romulan Republic. The crew of the ISS Excelsior who had exchanged places with their normal universe counterparts encountered the creature as it left a trail of devastation in Romulan space. Hikaru Sulu nominated to destroy the creature as it was going to reproduce and multiply which would lead the space amoeba's to overrun the galaxy. Similar to the primary universe, the Excelsior delivered an antimatter warhead into the cell nucleus of the organism thus killing it. This action earnt the gratitude of the Romulan people.