I really don’t know enough about each of the theories for the universe to give you a proper answer. I was reading up on the multiple universe theory and I found that very confusing to follow..
I think it could be possible that there are other universes out there. We humans have not travelled very far at all. Using the Hubble telescope we have seen much more than we have explored, and yet I doubt we’ll ever be able to fully explore space! We have a lot more research and engineering to do in order to be able to travel long distances in space!
I hope this is ok.. If you let me know what theory you believe in then I can see if I think it is believable to me too!