Question

Is there a another universe?

Answer 1

If there is, we would not know.

Explanation:

Linguistically the answer would be "no" simply because "universe" means "all existing matter and space".

In practice, when we started discovering that galaxies like the Andromeda Galaxy are outside our own Milky Way galaxy, the term "island universes" was used. There is some sense to this terminology in that the distance between our own galaxy and other major galaxies is so ridiculously large (e.g. about `2` millon light years to the Andromeda Galaxy which is in the Local Group), that any interaction with it will be somewhat limited in any reasonable time frame - unless we find some suitable "faster than light" mechanism.

Are there any really isolated universes, with which interaction would be impossible? There is by definition no way of knowing.

One interesting assumption implicit in the question is that "another universe" would somehow relate to time in the universe we know in some sense that we could use the word "is" meaningfully. There are enough problems with the notion of simultaneity in our own universe, let alone trying to relate it to any other.