Let’s pretend that you have a basket with 100 apples. You know apples are about 100g each, because you weighed 10 of the them and all of the apples seem about the same size. You know that basket weighs 1000g. You put the whole thing on a scale and find it weighs 500,000g. You know something else is in that basket. You aren’t sure what, and frankly it doesn’t make sense, but trying different scales and remeasuring more individual apples gives the same result. So you decide that there must be something you can’t see but must exist. That’s dark matter/energy.
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.