Or another way of thinking about it:

If you fire a particle at two slits with a screen on the other side you get an interference pattern on the screen implying the particle in a sense went through both slits and the results interfered. This happens even if you use quite complex particles such as C60 buckyballs. So the universe in some sense runs multiple versions some with the molecules going one way and some going the other. All the "multiple universe" explanation implies really is that this is a general phenomena and the universe runs all possible versions even when we can't observe the results.