Your question contains its answer. Volume would be an odd measure to use given it changes with speed (due to special relativity) and gravity (general relativit

Your question contains its answer. Volume would be an odd measure to use given it changes with speed (due to special relativity) and gravity (general relativity). The visible sky is over 99% empty, so it can't be that. Which leaves us having to choose between mass and energy. But as e=mc2, they are effectively the same thing, which means we don't have to choose.