The human race has searched far and wide for aliens and this may just be why we can't seem to find them. This is the dark forest.
This theory is based on the novel The Dark Forest. Imagine: you are an alien, part of a race that has colonised many planets. Like all life, you main objective is survival. You continue to go around to each planet in you Solar System and colonise it. Since resources in the universe are finite, all other life that uses these resources is a potential threat to your survival. The best way to guarantee you and your race's survival is to be the only ones using the resources. The best way to guarantee you and your race's survival is to kill all other life in the universe.
The universe is a dark forest. Every civilization is an armed hunter stalking through the trees like a ghost, gently pushing aside branches that block the path and trying to tread without sound. Even breathing is done with care. The hunter has to be careful, because everywhere in the forest are stealthy hunters like him. If he finds other life—another hunter, an angel or a demon, a delicate infant or a tottering old man, a fairy or a demigod—there’s only one thing he can do: open fire and eliminate them. In this forest, hell is other people. An eternal threat that any life that exposes its own existence will be swiftly wiped out.
– Liu Cixin, The Dark Forest
The quote “Even breathing is done with care” implies that aliens are trying to hide from other races to stay alive. The same applies for “An eternal threat that any life that exposes its own existence will be swiftly wiped out.”
The universe is a dark forest. And we are the hunters.