According to some theories, the galaxy should be overrun by technologically advanced extraterrestrial civilizations. The fact that no evidence of this has been found, is often referred to as the Fermi Paradox, which famously asks, "Where is everybody?"
In my view, this paradox is intensified by the fact that so far, very few earth-like planets have been discovered by astronomers, and even these seem to be very poor candidates for life. They are unlikely to contain technological civilizations.
It is not surprising that few earth-like planets would have formed by natural means--however, if there were alien civilizations, even a very few of them, they likely would have been able to modify the planets they found, so as to make them suitable for life.
This theoretical process is called, "terraforming," the altering of a planet so as to make it possess an atmosphere, water, and regulated temperatures, among other features of planet earth. Such planets would be expected to have stable orbits around stable stars.
Of course, if the aliens come from a planet unlike earth, and if they were capable of terraforming, they would be expected to prefer planets like their own. They would transform planets to meet their own standards and needs. If they did so, we would expect to see a class of planets that exist in large numbers, that closely resemble each other.
So far, this does not seem to be the case. We do not even find a large number of planets that meet the needs of any technological civilization.
Perhaps the closest we have come to finding evidence of alien technology is to observe "Tabby's star," a star which might be surrounded by an encircling cosmic structure being built. Reasonable conjectures have been made that the strange behavior of light from Tabby's star could be due to a project in process, a partially completed structure, designed to capture and use all the energy from the star.
Suppose that one or more stars became completely encircled by such hypothesized structures. Would they appear to us as very large, red giant stars?
In any case, we must consider that alien techno-civilizations might be very different from what we expect. Even if they began as we did, they might have followed a completely different path, especially once they made discoveries about the laws of nature that we cannot imagine, and applied them to technologies that not only far surpass our own, but are a different breed apart.
In our search for hypothesized alien civilizations, we should be searching not only for radio signals, but for "Tabby-like" stars, and planetary systems that seem to be customized. If we do not find them, then we must consider two alternative theories: either we are utterly alone, or else, the aliens are so far beyond us in advancement that we would not recognize them as such if we saw them.
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