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!)
Or is the solution that official science has just not been paying attention–to the craft in the sky or the reports from contactees on the ground. In other words, the Visitors are shouting from the rooftops, but all we hear is silence because we’re looking in the wrong places and disregarding the evidence all around us.
If we ARE being contacted, it’s likely that another intelligent civilization would send robot probes– this would be much cheaper sending astronauts into space. And interestingly enough, MANY contactees–including Whitley!–who’ve met the "Grays" say that they seem "machine-like."
Villard reminds us that if were built especially to remain unnoticed in our world, then "finding any trace of them would especially be a needle-in-haystack search. They would blend in with the natural tapestry of the solar system."