Letalium, also known as Letalium-666, is a very rare element with characteristics similar to uranium-235. 
Noone knows how these gluons can keep these enormous atoms from falling apart, but it is presumed Letalium atoms are held together by unknown subatomic particles which can only be produced within a black hole. For reasons unknown to scientists, these subatomic particles keep existing even after the black hole in which they are generated has been destroyed.

As the destruction of a black hole is an exceedingly rare event, this leaves Letalium very hard to find as no scientist has found a way to reproduce it yet.

Being much more potent than uranium, this raw material has much more potential when enriched and can be used in energy production as well as in new weapons.