I think M-Theory is the lastest theory, an evolution of String Theory (String Theory came up with 5 theories, each one valid)
M-theory has combined the 5.
Its interesting, these theories predict the existance of extra diamensions (M-Theory sets the number at 11).
M-Theory involves Membranes of universes, and Gravitrons travelling to other membranes through the other diamensions. (its trying to explain why gravity is so weak [we're losing some gravitron particles to these other universes])
If you can manage to get a copy, read Elegant Universe by Brian Greene