Energy is never created or destroyed, only converted into different forms (although one of the forms it can be converted into is matter).
If you're trying to figure out where energy "goes" once it is "used", it is converted into forms which are not reusable. The canonical answer for where the energy "goes" that I usually hear is that it becomes "heat". Probably a better answer is that the energy doesn't "go" anywhere, it is just continually converted from lower-entropy to higher-entropy forms ("higher-entropy" effectively meaning "less useful").
Dark Energy is, by definition, the expansion above and beyond what would be caused by the actual "regular energy of the universe being dissipated in space". If it were regular energy dissipated in space it wouldn't be Dark Energy, it would just be Energy. Right? You don't even have to explain what Dark Energy is to say that much, it's just the semantics of the thing.
If you can explain what dark energy is they'll probably give you a nobel prize.