It is funny, you get triggered by 'just a theory' but then apply the same dismissal to other things that are at least theoretically not impossible.
It IS just a theory, Einstein's relativity.
But not in the sense you seem to take it, that that means it is not relevant or right. But in the sense, that it is just one point in time and science, has replaced previous equally at the time applicable theories, and will itself be replaced.
It was theoretically impossible to have elctricity powered homes, before we knew what electricity was as a concept. It was theoretically impossible to have diesel powered cars rather than wind-up mechanics, before we knew what oil was. It was theoretically impossible to split an atom, when we didn't know what an atom was. Or to escape gravity, before we defined the rules of gravity. And before we could quantify and understand the speed of light as a theory, that itself was irrelevant to non-existent to us. Matter itself did not exist as a concept, until it did, and then the idea of being able to create mass, was a fictional concept. Until it wasn't.
I could go on and on and on, and I refuse to believe you don't see my point.
One day we will develop a concept of something not understood up until that point, and it will change things and shift parameters of possibility, as it always has. Something that might seem easily dismissable or outright unthinkable now, but once it becomes theoretical, becomes an avenue to explore. It is inevitable. Otherwise we would just stop teaching and practicing science. Elsewhere on the universe as big as it is, this may or may not have already happened. But to suggest it can't happen here, is just bloody pointless and pigheaded, and goes against what we have as a species been doing since we came to exist.