Dark energy and dark matter actually belongs in that area of mystery. Everything without solid proof is mere theoratical or hypothetical. Even with what we know now, there are limitless of things there is yet to be discovered and learned. Basically we know nothing. We are just observing.

They have found dark matter/energy. They just dont know what is their make up yet. Like what cherroy said, it is responsible for the expanding of the universe.

And yes, perhaps physicists are wrong somewhere in their calculations but it is part of the journey of discovering the truth - by making errors. They are right many times though, otherwise u will not be flying on planes or posting in forums here thru the internet.