The idea that all possibilities "actually" happen can realise no greater possibility than the situation that is described by what we imagine might "actually" happen.

 

It follows that if we can't imagine the actuality of ONE possible world, then we cannot support "any" possible world as being actual.

Well since all elementary particles have a wave function, and the universe was just a single point, it had a wave function. If you apply the wave function to that single point of the universe, you essentially get a map of all the probable universes. So where are they you might ask? They are around somewhere, but we don't have the capability to see them. They are like ghosts, but if you were in any other universe, then this universe your in now would be a ghost to that universe as well. This is because they are also still technically entangled as well.