Basic Information
Man ventures out into space looking for intelligent life, but finds none. There might be alien flora or fauna out there, but otherwise we are alone in the universe. This does not rule out robots, genetically engineered monsters, and colonists who long ago branched out into a human sub-species however.
Naturally, there are consequences to this - if life is plentiful but intelligence rare, this makes humanity very special indeed. Why has only one species in all this abundance become sapient? If life is rare, this has similar implications for humanity and, by extension, Earth. There are, of course, other possibilities - a common idea is that something is killing off intelligent life. What is it, and why did it miss humanity out?
This is distinct from the trope where humanity finds no aliens, but does find the footprints they left behind - such as ruins, robots, genetically engineered monsters and human colonists abducted from earth long ago. These sort of aliens are known as precursors and come in a variety of flavours.
A further subtrope is one in which we are the precursors and the whole campaign takes place After The End of some galaxy spanning human empire that we all forgot. In this case we may also be the force that wiped out all the aliens.
See Also
Sources
http://tvtropes.org/pmwiki/pmwiki.php/Main/AbsentAliens
Game and Story Use
- This does not rule out other types of non-human beings. Robots, genetically engineered monsters, uplifted animal, e.t.c
- A starship crashes on an uninhabited planet, where the crew must contend with some rather nasty native life and internal disputes while trying to survive until they are rescued.
- Without aliens, the interactions between humans and other human gets a lot more emphasis. They explore space, establish settlements on far-off worlds which eventually grow to be interstellar civilizations in their own right. What results is an eventual First Contact between two long separated human lineages.