According to one strand of theory, there are any number of universes,
self-contained separate realities, coexisting parallel to this one we know.
Supposedly they are all very similar, but with small differences. One wellknown
example is of a universe where Barbara Bush is married to Saddam Hussein.

If every action, for example a marriage between two people, opens the possibility
of another entire reality, then there must exist an infinite number of these
co-existing universes. If every action carried out in this universe can also
be carried out in any different number of ways in other universes, then
ultimately you, in summation of all these realities, are everything.