The materials that make up life are everywhere [in space]. Water is one of the most common molecules in the universe, and the light elements - carbon, nitrogen, and oxygen - are ubiquitous. Many experiments have shown that from these very simple materials you can create the organic molecules for life. It almost, then, becomes a statistical matter: You put the right materials together with an energy source, and things are going to regurgitate and percolate. Eventually the chemistry becomes complex to the point that it begins to control its own future. Then a self-replicating molecule comes into the picture. It is virtually unimaginable that all those stars resembling the sun could form and leave an environment around them so clean there's nothing left to coalesce into smaller 'coaly' bodies. So there must be planets around. Many stars will have planets too close, too small, or too far away from them. But those with planets at the right distance will have liquid water on them, and once you have liquid water and all the other stuff, life is going to happen.
