Fermi Paradox was first pointed out in 1950 by the physicists Enrico Fermi and Michael Hart based on probabilistic arguments about the existence of alien civilizations, such as the Drake Equation: if there really is a great amount of technologically advanced extraterrestrial civilizations, as estimates seem to predict, why until now haven't we seen any direct evidence of the existence of even one of them? This becomes even more problematic if we consider that several star systems, and even an entire galaxy, could be colonized in a few million years, a very short time that should have already passed: if even one advanced civilization ever existed before us in the Milky Way, we should expect the entire galaxy to belong to it by now.