In Asimov's Foundation, immediately following the exile of the encyclopedists, the envoy of Anacreon is tricked into disclosing critical information. The mayor at Terminus asks whether they could supply plutonium. The envoy is unaware that apparently, heavier or other elements are used in nuclear fission plants. They conclude that Anacreon and other stellar kingdoms are running on "oil and coal". They also say nuclear power is 50,000 years old, presumably this intergalactic future may be 50,000 years ahead of our present era.
Coal and oil on our present Earth is the result of millions of years of sedimentation of organic matter. Does that mean that either Earth continues to produce these resources, that they can be synthesized, or that alien life exists? If the latter, why are there so far no other alien species of mention?