The Large Magellanic Cloud was a galaxy located in the universe within relative proximity to the Milky Way.
It was James T. Kirk who first theorized that flying parasites discovered on Deneva may have came from "an entirely different galaxy," during the encounter there in 2267. Spock further speculated that this other galaxy may be "a place where our physical laws do not apply." (TOS: "Operation -- Annihilate!")
By the late 2360s, the Large Magellanic Cloud was speculated by Federation scientists to be the possible origin of the flying parasites discovered on Deneva. (DS9: "The Nagus" okudagram)
The fact that this is only the possible origin is suggested due to the okudagram listing the reference as "Large Magellanic Cloud ?".