I mean... isn't that the point of dark matter and dark energy? To be a stand-in name for stuff we have to account for but can't detect properly? The dark here means dark as in "we are in the dark about that".

It does not really matter if those exist or are an artifact of current theories, replacing relativity is not that easy and there are already a ton of physicists working on that anyway.

In the meanwhile of either having instruments capable of detecting these or a new theory that demostrates that those are artifact emerge... they have to exist. That's the point of them.