Dilithium

Dilithium is a rare crystalline element, found through out the universe. Dilithium is the only known material that can regulate the matter/antimatter reaction, which is why it is used by most species in their warp drives.

The planet Direidi is one of the most concentrated locations of Dilithium in known Federation space, and is located on the border to Klingon space. (TOS novel: "How Much for Just the Planet?")