What is the Universe?
The word Universe generally means “All of space, all of time and all its contents”. In other words the Universe consists of absolutely everything that exists in the past, present and future.
By this definition there can be only one Universe. There is no such thing as “outside” the Universe because it literally means every place, every time and every thing—including any places or dimensions we don’t yet know about.
Note that, although this is a good general definition, the reality may be more complex (see The Multiverse).