Space amoeba

From Memory Alpha, the free Star Trek reference.

The space amoeba was a vast, space-dwelling being, single-celled organism of unknown origin, surrounded by a field of negative energy.

The USS Enterprise encountered the space amoeba in 2268 while en route to Starbase 6, after diverting to investigate the destruction of an inhabited planet in the Gamma 7A system and the all-Vulcan starship USS Intrepid.

The Enterprise crew discovered that the negative energy field was toxic to humanoid life. They also found that the organism would soon reproduce by cell division. In view of the danger posed by the organism and its multiplication across the galaxy, the Enterprise crew used an antimatter charge to destroy it. (TOS: "The Immunity Syndrome")