Exonium was a fossil fuel that could be used to produce energy. The planet Rishi was rich in deposits of this mineral. The native Rishii used the mineral in food preparation. More developed cultures used the mineral as power sources for things such as sonic weapons. Exonium could be refined in a way that made it a suitable fuel for use in starships.
During the Cold War and Galactic War, pirates used Rishi as a base, taking advantage of the exonium deposits to provide them with a fuel source that did not rely on the normal galactic economy. Exonium was produced on the neutral space station Mek-Sha.
During the Clone Wars, the Separatists invaded Rishi in order to obtain control over the exonium mining there. Kit Fisto lead a group of clone troopers into a battle to regain control of the mines.
-Wookieepedia
Ingame Information
In Space Engineers, Exonium is more like Plutonium, being used in Fusion Reactors. It can be converted into Uranium. Exonium is the best fuel for fighters because it doesn’t need Tanks like Hypermatter and produces more energy than Synthetic Fuel.