We still have no idea what carries the force observed as inertia. While it may be a hypothesis that aliens exist, it's also a hypothesis that they don't. We know things have inertia, but have no idea why they do. We have a reasonable assumption that aliens exist. Proving they exist consists of finding an alien civilization, but proving they don't would include an exhaustive search of the entire universe. Inertia appears to be a universal force, but proving that would also require an fairly exhaustive search from everywhere in the universe.

It's a bit more than opinion, it's the Cosmological Principle:

https://en.wikipedia.org/wiki/Cosmological_principle

There's room for debate but it generally seems to match our observations. There's no a priori reason to believe that Earth is special in the universe since all the rest of our solar system / galaxy seem to be unremarkable.

Sure, but if we're going to look let's look in those places that are generating the kind of evidence we'd expect to see which are showing interesting behavior.