Mass can neither be created nor destroyed. Energy can neither be created nor destroyed. And we know from observation of an object exploding that there is less mass after a release of energy. From this can we assume mass is stored energy? Or is mass transformed from this abstract substance into another abstract substance? Are they equal in value or two forces interacting? Before you respond let me ask you a few off topic questions. what do the curvature of a circle, the parallels of a right triangle, and many other observations of existence have in common? How do they differ from Euler's identity?
E=mc^2 expanded or the theory of movement.
They are similar in their interactions. I don't know why. But I can show you how. Maybe how is all we can know.