It is known that are an infinite number of worlds, simply because there are an infinite amount of space for them to be in. However not every one of them is inhabited. Any finite number divided by infinity is as near nothing as makes no odds, so the average population of all the planets in the universe can be said to be zero. From this it follows that the population of the whole universe is also zero, and that any people you may meet from time to time are merely products of a deranged imagination.
Now logic is a wonderful thing but it has, as the processes of evolution discovered, certain drawbacks. Anything that thinks logically can be fooled by something else which thinks at least as logically as it does.
Source: The Ultimate Hitchhiker's Guide to the Galaxy