Here’s a short from 2015 from The Economist: “Do We Live in a Multiverse?”
The video (9 min) presents the theories of MIT professor Max Tegmark.
Space continues to expand, so we can see about 42 billion light years right now from Hubble.
There are four possible types of multiverse.
The first is simply that space keeps expanding wherever there is minimal gravity (or enough dark energy).
The second is that it breaks off into “pocket universes”, each of which can have its own laws of physics. We happen to live, according to the “anthropic principle”, in a universe where life can work. It might be that the pocket universes all satisfy different expressions of string theory.
The third is one where universes are separated by time, and where reality may fork off into new universes, and that we personally might progress through them without realizing it. (But that would imply any parallel universes would have to follow the same laws and constants in physics.)
The last is simply the “mathematical multiverse”.