The problem with the universe is that it's big. Not just large in terms of physical scale compared to the domain that humans occupy, but large in terms of the sheer numbers of atoms, molecules, asteroids, planets, stars, and galaxies. This means that if you want to look for a few special things out there in the cosmos you have to spend an awful lot of time and effort just sifting through everything else.
There is perhaps nowhere that this challenge is more starkly evident than in the quest to see whether or not there are other technological species out there in the universe. The numbers are staggering. Our galaxy, the Milky Way, harbors by some estimates as many as 400 billion stars. We now suspect that most will host planetary systems. If any of those places sustain complex, communicative life then there are an absurd number of channels by which they might - either deliberately or inadvertently - betray their presence to the rest of the galaxy.
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