Since I define the Universe as the total of everything that exists, there can’t be other Universes.
I'm of the opinion that I'm the only thing that actually exists. Sorry.
The Universe is that collection of points and particles
that can in principle interact with each other
by exchanging some particle (photon, graviton, etc.)
By definition in a “Multiverse” (whatever that is)
The collection of points are not able to interact
at any any level, and as such are separate Universes.
And are "unknowable" to each other
The Multiverse concept is a null hypothesis
as any other universes cannot interact with this one we are in, in principle.
Other Universes are Unknowable