Fission and fusion can convert mass to energy, what is the mechanism for converting energy to mass?

Question

Has it been observed? Is it just theoretical? Is it one of those simple-but-profound things?

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In: 3715

Answer 1

Fission and fusion take an input of x mass then something happens and the output is (x minus a tiny bit) mass. This tiny bit is converted to energy via E=mc^2.