However, if the universe is swarming with life, and if a significant percentage of that life is intelligent, why is the universe so quiet? This is the essential conundrum behind the Fermi paradox; the observation that while intelligent alien life may be statistically or even astronomically high, it is utterly devoid of signs of life.
There may be a number of reasons behind this great silence. It could be that humans haven’t been listening long enough or know what to look for. Civilizations could have fell in their infancy after the intelligent species went extinct. They might have killed themselves in a planetwide war or got wiped out by asteroids. One eerie possibility is that something or someone is out there culling the numbers.
Speculative fiction often depicts unmanned alien probes sent to explore space and seek out signs of intelligence. One, the so-called Von Neumann probe (named after the mathematician John Neumann) self-replicates in order to seed itself throughout the universe. One popular and less friendly variant of the alien probe concept has also become a trope. After finding proof of intelligence (usually through some kind of test), a doomsday weapon type of probe could promptly annihilate that life – leaving one less intelligent race in the universe. An interpretation of the concept was central to the Berserker series of Fred Saberhagen.
The creators of such a probe could be as advanced as they are cruel. Perhaps the protagonists rush to answer a series of riddles like the mythical sphinx in the hopes of getting a reward. Maybe it’s armed with an array of powerful and advanced weapons that the military thinks they can disarm and reverse engineer. Perhaps it even has some form of intelligence and, at the zero hour, spares its target out of sympathy.