There is some interesting stuff on this site: Seven Dimensional (and up) Einsteinian Hyperspherical Universe.
From briefly skimming the material, 7 dimensions is from two 3D 'universes' bound with a common time. I think I remember Hawking mentioning something like this, making the analogy of a 'balanced equation'.
I find it frustrating that much of this sort of material doesn't contain dates, so when I read about scientific theories I'm never really sure if that is 'still what the scientists think'. I'd love to know of a site that had 'current theories' listed. For example, I haven't been able to determine if people still believe in entropy..