Fermi's calculation was that it would take a civilization between and 1 and 100 million years to colonize a galaxy at sub-light speeds. There has been ample tim

Fermi's calculation was that it would take a civilization between and 1 and 100 million years to colonize a galaxy at sub-light speeds. There has been ample time for multiple civilizations to have done this. So where is everyone? If there's been on average a thousand civs spread across the Milky Way for the past 5 billion years, then we need something else to explain why none of them have colonized the galaxy.