Our Milky Way galaxy is so big and so old–it contains at least 100 billion planets–so aliens should have visited us by now (Whitley Strieber thinks THEY HAVE!) This is what’s known as the Fermi Paradox.
In Discovery News, Ray Villard talks about science fiction writer Karl Schroeder, who has come upon a solution to the Paradox. He thinks that aliens have "gone green" and generate no waste products that we can detect. They therefore blend into the galaxy. Villard quotes him as saying that, in their case, "artificial and natural systems are indistinguishable." Villard theorizes that maybe only ecologically-balanced civilizations survive in the long run (that means WE won’t last long!)