What is beyond our universe?
It seems to me that there are two problems with this question:
1. The meaning of the word universe.
Universe literally means all existing matter and space.
So that excludes any other existing matter and space. Any existing other matter and/or space would fall under the definition of this universe and so be part of it.
2. The word is.
If it is meaningful to speak of something beyond our universe, then what is its relation to our universe in terms of time? There are enough issues with the notion of simultaneity within our universe, let alone in relation to something beyond our universe. So what does the word "is" mean? If something "beyond our universe" does not interact with our universe, then in what way is its time related at all? If its time is not related then there is no meaning to "is".
Having said all of this, note that historically when we discovered the existence of galaxies outside our own Milky Way galaxy, the term "island universe" was used for galaxies. There is some practical value in this usage since the distances between our own galaxy and some of its more significant "local" neighbours (i.e. discounting the smaller satellite galaxies) are of the order of