I think they can also just be an unknown phenomena. They don't need to be literal mass or energy. Essentially, to fit our current model to observations of how our galaxies and the universe in general behave, we need to add mass and energy to the equation to make it match. Essentially, dark matter and dark energy are fudge factors to get our models to work.

There are a ton of theories to reconcile the differences, but very few are provable with our current techniques. Detecting literal dark matter is one possibility. Changes in universal constants would be another.