cern,
that is only one theory among many - and it is becoming rather outdated now.
To view the Universe as a bubble is in fact a very simplified way to see it. It is easy to argue that the _observable_ Universe has the shape of a sphere (because we find ourselves in the middle of it, and can see equally far in each direction in spatial 3D) - but you cannot from that simply conclude that the entire Universe has a spherical shape.
Here's a simple
MIT Cosmology Faq which addresses the very question which started this thread.
Tormod
