Fermi paradox (E. Fermi):
E. Fermi's conjecture, simplified along phrase, "Where are they?" questioning that if the Galaxy is filled along with technological and intelligent civilizations, why still haven't they come to us? There are many possible answers to this question, but as we only have the vaguest idea what the correct conditions for life & intelligence in our Galaxy, it & Fermi's paradox are no more than speculation.