The idea, borne of the overconfidence effect, that the universe must be teeming with aliens and the subsequent disappointment that there is no evidence of this. Actual analysis of the Drake equation with reasonable values produces a very slim likelihood of alien life, if it exists, of being visible -- realist values seem to produce less than one civilization per galaxy. Still, assuming there are many friends or enemies hiding just around every corner produces excellent science fiction, so it remains popular.