Look, we write rather a lot about the Fermi Paradox, so trust us when we say that the Berserker Hypothesis may be the darkest explanation out there. Not only would it mean that the universe is a dead, lifeless husk, but it would also imply that our own destruction is imminent.
The Fermi Paradox at its most basic is, given the high probability that alien life exists out there (bearing in mind the vastness of space and that we keep finding planets within habitable zones), why has nobody got in touch yet?
If there are so many other civilizations out there – possibly at far more advanced stages than we are because of how long the universe has dragged on – surely at least one would send out probes, or is actively trying to inhabit the galaxy? If so, why haven't we come across them? Anyone who believes there is life out there in the universe has to explain what has become known as The Great Silence from any advanced civilization out there.
Adding to this problem somewhat, mathematician John von Neumann proposed a type of probe which would become known as the von Neumann probe. The idea is that once a civilization is advanced enough, they could send out self-replicating probes into the universe. These probes would harvest energy from stars, and mine planets and moons for materials, to build more probes that will do exactly the same, like seeds propagating throughout the universe.
As Michio Kaku explains below, these probes could even take the form of nanobots.
Various estimates on how long it would take for von Neumann probes to populate the galaxy or the universe have been put forward, with some suggesting that the process could be quick enough to warrant the question: why on Earth haven't we seen any signs of them?
Carl Sagan's answer was to assume that any civilization advanced enough to make the probes would not be dumb enough to actually make the probes.
"These implacable replicators will not stop until the entire universe has been converted into ~1047 von Neumann machines, which then presumably cannibalize each other," Sagan wrote in a paper. "If anything like this were a real danger, an emerging interstellar civilization would be wise, as a matter of self-preservation, to take steps to prevent it."
"The prudent policy of any technical civilization must be, with very high reliability, to prevent the construction of interstellar von Neumann machines and to circumscribe their domestic use."
What's more, civilizations would also seek out and destroy any other replicating probes they came across.
Others, however, have not ascribed advanced civilizations with a desire to protect the universe. There have been suggestions that dying aliens could create the probes as a way of preserving records of their own achievements for future civilizations, or to seed life throughout the universe. But what about the aliens, be it one species in 100,000, that want to use them for malignant purposes?
The Berserker probe, first outlined in science fiction by author Fred Saberhagen, is a theoretical probe sent out as a way to protect the creator species (though in some variants, the Berserker probe is simply a von Neumann probe that has gone insane, and replicates with the goal of destroying other civilizations, or merely destroys them while harvesting their resources). The probes would seek out and destroy developing civilizations before they can pose a threat.
"Let us say many advanced ETIS [extraterrestrial intelligent species] get the robot-emissary idea and ship out first-generation probes...to replicate and fill the void with messages of brotherhood. Then suppose that for every 100 or 1,000 or 10,000 'sane' ETIS, there is one that is xenophobic, paranoid even. Such a race might program its self-replicating emissaries to add powerful bombs to their repertoire, and command them to home in on any unrecognized source of modulated electromagnetic radiation," NASA scientist Glen Brin expanded in a paper on the topic.
"It need only happen once for the results of this scenario to become the equilibrium conditions in the Galaxy. We would not have detected extra-terrestrial radio traffic – nor would any ETIS have ever settled on Earth – because all were killed shortly after discovering radio."
Should this theory be correct, it would mean the universe is silent because whenever a civilization is "loud", they soon become transformed into "dead".
Though presumably more as a thought experiment than an actual proposal, Brin suggested that previous extinction events on Earth – and the large intervals between them – could be accounted for by the time it takes for a civilization to spread Berserker probes out through the galaxy.
"If the ecological holocaust of the Cretaceous was a local manifestation of the death spasm of a prior space-faring race whose overpopulated sphere of settlement spoiled as the shelll of 'civilization' passed outward, Earth may be the first nursery world in the vicinity to have recovered sufficiently to develop a species with technology," he concludes, adding "it offers an explanation as to why our region of space might be in a disequelibrium state of emptiness".
Or perhaps another Berserker probe, triggered by our own signals haphazardly sent out into space, may already be on its way.