There is a terminology problem here. Sometimes we use the term
"universe" to refer not to everything there is, but rather to the full
four-dimensional space-time structure we see around us. Using this
terminology there could be other universes, and in fact we read about
"baby universes", "basement universes", etc. Sometimes people then use
terms like "multiverse" or "omniverse" or other coinages to refer to
the collection of all the universes.
If you want to reserve "universe" to mean all that there is, then you
need to suggest a word for the four-dimensional space-time where we live.