I know that energy can convert to mass and vice versa.
So are we not looking at simply two forms of the same thing?
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Einstein's great realisation (or one of several !) was that energy and mass are the same thing, just (sometimes) measured in different units. There is no conversion involved (unless you mean a conversion of units). The equivalence is made transparent in particle physics where the electronvolt is both a unit of energy and a unit of mass.
"Mass and energy can be seen as two names (and two measurement units) for the same underlying, conserved physical quantity" - Wikipedia Mass-energy equivalence