That's that funny thing, they've tried different scales. They've tried radically different ways of measuring it, and always come up with the same discrepancy.

If summing energy works differently on a large scale, why? Since we don't know what we can do is start measuring the difference between observable energy and the "extra" that appears when we add it up. We could call that "unobservable energy" so we can see if there is a pattern, or if it's actually something else. You know "unobservable energy" is a mouthful, why not just call it dark energy?

We don't know what it is. We have tested lots of theories and dark energy doesn't seem to fit any answer, hence the name. I get thinking that it can't be that hard to reconcile and scientists must be missing an obvious conclusion, but it's likely that your theory has already been tested. Maybe you have the solution and can resolve the discrepancy, but right now all data shows that dark energy is a large part of the universe.