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Why have they not shown up? Fermi's Paradox is what started the question in the popular consciousness but I'm sure many others contemplated the same thing long before. Essentially, if a species can colonize the entire galaxy in a matter of millions of years by employing the use of self-replicating machines or something similar then WHERE ARE THEY? Note that this does not require faster than light travel. It only requires vast numbers of self-replicating autonomous entities, mostly coordinating their activities to colonize as many star system as possible.