Some people stick with the traditional, feeling struck by the epic beauty or blown away by the insane scale of the universe. Personally, I go for the old “existential meltdown followed by acting weird for the next half hour.” But everyone feels something. Physicist Enrico Fermi felt something too—”Where is everybody?” It turns out that when it comes to the fate of humankind, this question is very important. Depending on where The Great Filter occurs, we’re left with three possible realities: We’re rare, we’re first, or we’re fucked.
The Fermi Paradox poses the question, if intelligent ETs exist why haven't they shown up yet. Now a new mathematical study shows that self-replicating probes using sling-shot maneuvers (paper) could explore the entire galaxy in just 10 million years. Perhaps they're already here, hiding, waiting for our technology to reach a level that can de-cloak them.
David Brin points to Asteroid belts at just the right place are friendly to life -November 6, 2012 , saying "This combination is calculated to be rare, in perhaps just 4% of solar systems. That rarity offers yet one more new, rather daunting candidate for the Fermi Paradox."
The "Great Filter" is a hypothetical barrier to explain why civilisations are so unlikely to progress to the point of inter-stellar colonisation that we have not encountered any in 40 years of looking. Maybe humanity has already negotiated the filter - as some massive evolutionary improbability - or perhaps it lies in our future as an almost-certain threat to our existence? We should hold our breath as we look for evidence of life on Mars.