It's just another bit of the Universe.
Other Universes - if they existed - would have to be strictly orthogonal to our Universe, and thus be undetectable.
Whenever we detect something - whatever we detect is by definition part of our Universe.
If/when we find different phase spaces, areas of reduced or enhanced dimensionality, multiply connected space - whatever weird and wonderful things are out there - we're merely discovering that our Universe is more complex and strange than we originally thought.
Other Universes cannot be detected. If you can detect them then they weren't 'other' Universes in the first place.
There is no observation you can make that can show the existence of another Universe. If you can observe it, it's in our Universe.
The theory of a 'MultiVerse' of multiple Universes is not only non-disprovable: it is also strictly non-provable. This makes it a uniquely pointless concept.
The Universe's parameters have to be exactly fine-tuned to allow for the existence of life. The fact that they have been so fine-tuned argues for the existence of a Creator.
Multiverse theory is not scientific: it's a figleaf.