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"For example, nanotechnology may facilitate the development of Von Neumann probes. As physicist Richard Feynman observed in his seminal essay, “There's Plenty of Room at the Bottom,” there is nothing in the laws of physics which prevents building armies of molecular-sized machines. At present, scientists have already built atomic-sized curiosities, such as an atomic abacus with Buckyballs and an atomic guitar with strings about 100 atoms across.
Paul Davies speculates that a space-faring civilization could use nanotechnology to build miniature probes to explore the galaxy, perhaps no bigger than your palm. Davies says, “The tiny probes I'm talking about will be so inconspicuous that it's no surprise that we haven't come across one. It's not the sort of thing that you're going to trip over in your back yard. So if that is the way technology develops, namely, smaller, faster, cheaper and if other civilizations have gone this route, then we could be surrounded by surveillance devices.”

Furthermore, the development of biotechnology has opened entirely new possibilities. These probes may act as life-forms, reproducing their genetic information, mutating and evolving at each stage of reproduction to enhance their capabilities, and may have artificial intelligence to accelerate their search."

This argument, plus the arguments in Rare Earth, leave little doubt that we are alone at least in this galaxy and perhaps in the universe. I would put the upper limit of intelligent species in the galaxy at no more than 100 and possibly no more than ten.

Otherwise we would be having visitors (large size, the guys who built the nanorobot probes).

The Fermi Paradox.

--Boris

I don't think I completely agree. Machines fail over time, and self-replicating ones would have replicating errors over time. Also, why would a race use the extremely valuable resources of their solar system to create one-way journeys that would give zero return for their effort?

Maybe dark matter is more than subatomic particles.

Rare Earth

Definitely went against the grain when it was published. However, it makes a lot of sense. A lot more than most wishful thinking that is going on in this area.