I've told this a lot of times before but no matter, I'll tell this again (seeing as we have quite a few new members on board and all): the Finnish astronomer Esko Valtaoja likes to say that the Big Bang occured at the tip of his nose. You can see it in this pic:
Otherwise, it's as Slang said. If you are thinking about the famous balloon analogy, then the point from which the balloon expands is a point in time (the lowest possible limit to which is known, probably fairly accurately) and the surface of the balloon of a 2D representation of the 3D (as in spatial dimensions) universe.
The dog, the dog, he's at it again!