I've just realised why there are no "little green men" or UFOs.
Like us, other intelligent lifeforms have evolved their space traveling technologies incrementally.
However, by the time they get to a technology good enough for pan-galactic travel their locality is so clogged up with space junk (satellite fragments, old boosters, flecks of pain etc.) that they can't get out of their system and are doomed to wait for their sol to turn super giant or go nova.
  