A multiverse would be the base material for all of the individual universes to form in. It would have the characteristics of some simple medium that generated laws of physics based on geometry or topography or some other property. The laws of physics would vary in the universe of universe. Some, like ours would be stable. Others would disappear due to instability. Ours may be the only stable one.
The ToE meets cosmology, eh?
A couple problems with the ‘multiverse’ theory:
1. It’s an obvious cop-out to avoid the laws of probability. It leads to absurd conclusions if applied consistently. For example, it means that somewhere out there amidst an infinite or near-infinite pile of multiverses, perfectly formed dollar bills really do grow on trees. All the events of The Hitchhikers Guide to the Galaxy series are literally true, somewhere. It doesn’t matter how low the odds are, with enough multiverses out there you will find anything eventually.
2. There’s not a shred of evidence for them.
The preference for multiverses is thus purely emotional in nature, out of a desire to avoid God. As far as the nature of God goes, Romans 1:20 is clear enough, and His law is written in our hearts. Those who self-righteously try to judge Him by citing evidence of evil in the universe miss the point that we can only recognize evil because we have God’s moral law inculcated in us to begin with. If there is no God then there is nothing to complain about, the universe is just what it is and morality is bunk.
Sounds good, until you get to the point of trying to define each of those concepts in a way that is not tied to this universe.
What does "medium" mean? What is a "base material?" Where did it come from?
Beyond that, you still haven't told us how they got there in the first place.
You're just speculating: making some assumptions about how you think it might be.
So before you go feeling all proud of yourself for stumping those silly folks who would posit a "Creator," you might give some attention to the fact that you're in the same boat.