About alternate universes, realities or time lines. Considering they technically don't exist in ours wouldn't it be impossible to prove that they even exist?
Most likely, but that would really depend upon the specifics. I wouldn't count on it ever being settled conclusively. At this stage I wouldn't expect it to be testable any time soon.
There's also the issue of identifying whether it's the universe itself that's inconsistent, our flawed observation or just a complicated enough set of rules to make it worthwhile.
That being said, it isn't necessarily the case that we can't test for it, it's just that nobody has figured out how. Multiple time lines and realities would likely leave at least some traces on what we know. And there's an awful lot of weird stuff that happens.