As i understand it with my consciousness somewhat limited by matter and innerstand it with my sub-conscious:

It's not just the body's energy content alone, but how this reacts with external forces, known or unknown, which will affect both the mass and the energy content of any body.

For years thought was that there was nothing at the beginning, but as we know, you only get nothing from nothing, so the something that exists is what we are calling dark matter and dark energy, which affects all energy and matter in this universe.

Therefore the attempt to calculate this effect by using and expanding on Einstein's conclusions, who himself was only expanding on that which went before him.