Science can't prove all true things, or even prove anything with absolute certainty, but you can get closer to the truth through science. Some things might not be knowable through science, but if they aren't knowable, they probably wouldn't have a real world use. If parallel universes exist and there's no way to access them or prove they exist, then it basically might as well not exist. It would make no difference if there are no consequences of it being real or not. Unless there are consequences of something existing, something we can do with the knowledge, it only satisfies our curiosity. It sucks, but that might be the practical answer.