If the galaxy is old enough for aliens to exist, where is everybody?
The Milky Way holds over 100 billion stars, is roughly 13 billion years old, and almost certainly contains countless Earth-like planets. Even at sluggish sub-light speeds, a single spacefaring civilization could colonize the entire galaxy in a few million years, a blink of an eye on cosmic timescales. Physicist Enrico Fermi reportedly asked over lunch around 1950: if intelligent life should be common and should have had plenty of time to spread everywhere, why do we see no trace of it at all?
Reveal the answer
There's no agreed answer, only a long list of competing explanations: intelligent life might be far rarer than assumed (the 'Great Filter'), civilizations might reliably destroy themselves before becoming interstellar, interstellar travel might be far harder than imagined, or advanced civilizations might simply have no interest in contact or expansion. The gap between how likely life seems and how little evidence of it we've found is the Fermi paradox, first given a rigorous academic treatment by astrophysicist Michael Hart in 1975, long after Fermi's own remark went unrecorded.