According to string theory – a relatively new theory which attempts to combine all the forces known to physics into a single unified whole - there may be any number of universes in existence. However, each of them might be governed by different natural laws. For this reason most of these universes could not sustain life, since to find a combination of natural laws that would allow life to exist would be exceptional.
However rare such a combination may be, any universe containing an intelligent observer would inevitably be an exceptional universe of this kind. Some scientists believe this is one way of explaining why it is that the natural laws in our universe seem designed to allow life to exist because we are here to observe it. Or it may be merely a matter of chance that this is how it is.