Further to @Twisol's answer, consider the surface of a sphere - it is two-dimensional (if you're confined to the surface, you can only move in two directions) a

Further to @Twisol's answer, consider the surface of a sphere - it is two-dimensional (if you're confined to the surface, you can only move in two directions) and bounded in size, but it has no boundary as such; if you started walking around on a sphere, you could keep walking forever without hitting a wall. Now imagine there are two such spheres near to each other. Two beings whose physical laws and interactions were entirely confined to their respective surfaces would be unaware of each other and unable to influence each other in any way - gravity for example is only able to travel around the surface of the sphere, not into whatever 'space' lies in between. The surface of each sphere is its own universe.