OT - Physics Question on Anti-Matter

[Suicide Junkie]

nor nuclear fission; it's generated by the decay of natural radioactive materials like potassium, thorium and uranium

That's still supernovae residue, with the same ultimate source.

There is a modest amount of heat available from the formation and gravitational compaction of failed-stars and large planets. But again, that was all there in the potential energy of the starting material.