I'm no expert, but my understanding is a little different. Its not that energy "has" mass but that energy and mass are two different forms of the same thing, i

I'm no expert, but my understanding is a little different. Its not that energy "has" mass but that energy and mass are two different forms of the same thing, i.e. they are equivalent. If some process is able to convert mass to energy then you could find out much mass you'd get but multiplying the energy by c^2 (hence E = mc^2).