The Fermi paradox (1950) can be formulated as follows
If there are extraterrestrial civilizations out there, then where are they?
In a recent paper, Rasmus Bjork of the Niels Bohr Institute argues that finding other life in the Galaxy by using space probes, and possibly solving the paradox, might take extremely long time. A time which is comparable to the age of the universe. More precisely, he estimates that with 8 probes each having 8 subprobes around 4% of the Galaxy can be explored in 9.6 billion years.
So, if we sometimes feel lonely in the Universe it is because Aliens still haven’t had the time to visit us…
See also the discussion at Mangan’s Miscellany.