The Dark Forest Hypothesis. A very compelling answer to the Fermi paradox: If the universe is this vast and life surely must have developed over and over all around us, how come we never found anyone?
If two civilizations ever met, chances are incredibly slim that they were comparably or even similarly developed at this exact moment in time. Think about a modern army traveling back in time 400 years and fighting a group of swordmen with horses; the medieval people would be so overwhelmed it would barely classify as a fight, and that's just with a few hundred years of difference in technological progress. The random difference between species from different planets and systems would be far, far greater. So if two of them would meet, one of them would very likely be to the other as a god to an ant.
The universe might be brimming with life, but everyone who gets this far must be aware that half of them could wipe you out like ants, the other half could be as indomitable as a god. Cue the dark forest metaphor: There's prey and there's predators. We don't know which one we are in each instance, or how many of each are out there. But how could a first contact protocoll look like in such a competetive (and very likely deadly unfair) environment?
In the dark forest only two types of species can survive: Those that attack. And those who hide.