Dikironium

Dikironium is an element that by the 23rd century, could only be created inside a laboratory and was not believed to exist naturally. This was disproved in 2257 and 2268, when the crews of the USS Farragut and the USS Enterprise discovered that the dikironium cloud creature was partly composed of the element. (TOS episode: "Obsession", and comic: The Ashes of Eden).