What do you mean?

They didn't say it existed.

They just said the existence of Dark Matter seems to be more likely than non-uniformity of physics throughout the universe.

I don't think there is any sensible way of evaluating that.

Maybe they do.

Dark energy is a label for "whatever it is that causes the expansion of the universe which we have hard observations of."

Dark matter is a label for "whatever it is that causes the extra gravitation observed across multiple scales in the universe without also causing other indications of matter interaction like photon emission."

These are easy-to-hold handles for the collection of observations for phenomena we know to exist. We don't know the cause, but we know these things are occurring in the universe because we have demonstrated their existence through repeated observation.

I get your point, but "So far, dark matter/dark energy are in remarkable alignment with observations." is not in accord for "whatever it is that causes... <our observations>". It would be a tautology at that level, equivalent to saying "our observations are in accord with our observations".

Dark matter/dark energy work in the existing framework of GR and align well with observations. That's why they're so heavily favored. It's simple.

Saying the laws of physics may be different in different parts of the universe is a statement that so far can't be substantiated, and we've been trying for over a hundred years now, is far more problematic. That's a dire option that we don't desire to pursue unless we're absolutely forced to - and we're not being forced to.

Between the two options we're going to stick with dark matter/dark energy until evidence suggests otherwise, or until a better, and simpler, idea comes along.