The Fermi Paradox is far and way my favorite paradox:
An alien civilization could send probes traveling no faster than 10% the speed of light to every single solar system in the galaxy in only 10 million years. Which is incredible … As the researchers conclude … “a fleet of self-replicating probes can indeed explore the Galaxy in a sufficiently short time to warrant the existence of the Fermi Paradox.”
It’s Easier for Aliens to Visit Us Than Previously Thought | io9
Absalom, Absalom! 