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Originally Posted by eeyore  but for space to have any dimensions - and thus be able to curve, there must be something surrounding it to define the dimension |
Ah, Eeyore, you are still thinking in classical 3 dimensions - ie straight lines going on for ever. with curved space, "straight" lines will eventually end up at their origin. The curvature prevents anything leaving the universe and therefore prevents anything being outside of the boundaries of the universe.
As the universe expands its volume will increase as will the size of its boundary. (Analagous to the balloon being blown up. Except that nothing can exist or go beyond the boundary - light, straight lines, spacecraft or a God.)