There can still be an unobservable (to us) universe outside ours.
If you assume the universe started N billion years ago, there is an event horizon expanding at the speed of light that fit this definition of an "observable universe" for us while allowing for an infinite number of equal universes, some of which are partially shared with ours.
If you assume the universe started N billion years ago, there is an event horizon expanding at the speed of light that fit this definition of an "observable universe" for us while allowing for an infinite number of equal universes, some of which are partially shared with ours.