Dark matter is inferred and there is no concrete proof of it.
Correct. But then, the observations that made it clear that we need more mass to describe the motions of bodies in the Universe are fairly recent. Patience.
I note, however, that inference is very powerful. We infer your own existence from your postings, for example. Do you suggest we should also question whether you exist?
Almost by definition there can't be.
No. By definition our normal telescopes can't directly observe the stuff. That doesn't mean there can be no concrete proof of it - we simply have to use indirect observations, or tools other than telescopes.
It's possible that our theory of gravity is just wrong and variable gravity is correct (or one of a handful of alternative theories).
Yes, this is possible. Cosmologists don't deny it.
Theories stand (based upon observations) until they are disproven. They are not proven first. They do not even have to be replicated. Or replicable. Take for example the theory of multiple parallel universes.
You seem to be conflating the common language version of "theory" with the way the term is used in science. Among scientists, if an idea is untested, it is more usually referred to as an "Hypothesis". It becomes a theory only after the hypothesis is put through tests and found to be fairly solid.
There are wide swaths of hypotheses that cannot be tested, as they make no predictions. The hypothesis that there are universes outside our own is very nice, but unless you also posit that the boundaries between universes are crossed regularly in a particular way, or that the existence of another universe implies something specific that will happen in ours, then there's nothing to test. We can neither confirm nor deny the existence of other universes, as we have no way to check and see if they might be there.
Mind you, other universes are completely consistent with modern quantum mechanics. There's nothing in the way of them being there. But, the point is mostly moot, as we don't seem to interact with other universes in a meaningful way. Who cares if there's another universe, if you can't get there from here?