Is the multiverse theory plausible?

Sure. But whether it's provable is another question. We used to think the Milky Way was the whole universe. Then we found lots of other galaxies. Finding other universes is a bit different though. In theory we can never get out of our universe to go there. Anywhere we can get go is, by definition, still our universe. In this way our universe is like a black hole. No matter how hard we try we can't escape. Not even light can escape which means we couldn't see other universes if we wanted to. Their light is trapped inside them, like a black hole. So it would never reach our universe, never reach our telescopes.