SETI, the Search for Extra-Terrestrial Intelligence, has detected nothing. This result gives us "the Great Silence" paradox, which can be stated thusly: The size and age of our universe incline us to believe that many technologically advanced civilizations exist. However, this belief is in conflict with our failure to find observational evidence to support it.
Maybe intelligent life requires more than physics and chemistry and exists only on earth because a creator god of some sort put us here. Maybe technologically advanced intelligent life is rarer than we assumed. Maybe our current observations are misdirected or our search methodologies are flawed. It turns out that the last two explanations for this failure are both likely true.
Earth is located 27,000 light years from the center of our galaxy. SETI researchers look towards the center of our galaxy because that is the direction where the largest concentration of nearby stars are found. New evidence implies that long gamma ray bursts are more common in places where stars are more dense and also where elements heavier than hydrogen and helium are less common.
Long gamma ray bursts function like a reset buttons, they destroy multi-cellular life. Intelligent life, which we can define here as life that achieves an understanding of how stars give off light, is possible on earth because of our location on the outer periphery of a large galaxy where there are fewer nearby stars and where there is a significant quantity of heavier elements. Earth appears to have experienced a partial biological reset from a gamma ray burst resulting in the Ordovician extinction, a global cataclysm about 450 million years ago that wiped out 80% of Earth's species.
During the first 5 billion years after the Big Bang there were arguably too many long gamma ray bursts to make plausible the emergence of intelligent life. Furthermore, most galaxies are smaller than our Milky Way galaxy, with densely packed stars and/or with fewer heavy elements. It is estimated that about 10% of the observable galaxies are sufficiently large and with enough heavy elements to have planets in their outer regions with conditions amenable to the evolution of intelligent life. 10% of 100 billion is still 10 billion galaxies, but the large distances between galaxies reduces the probability of our finding evidence for intelligent life in other galaxies even when it exists.
Thus the initial SETI results appear to be not so paradoxical after all. Our chaotic universe provides a mostly harsh environment that is ill-suited for the evolution of intelligent life. The search for extraterrestrial intelligence may be more effective if it focused on the outer regions of our galaxy and nearby large galaxies, but this constraint also makes it less likely that we will ever find them.
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