The galaxy contains roughly a hundred billion stars. If even a very small fraction of these have planets which develop technological civilizations, there must be a very large number of such civilizations. If any of these civilizations produce cultures which colonize over interstellar distances, even at a small fraction of the speed of light, the galaxy should have been completely colonized in no more than a few million years. Since the galaxy is billions of years old, Earth should have been visited and colonized long ago. The absence of any evidence for such visits is the Fermi paradox.
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