Since the dawn of the Space Age, when it became apparent that traveling to other planets was technologically possible, humans have looked up and wondered if there were other intelligent species out there.
Yet in all of human history, we haven't found any inarguable evidence for the existence of alien civilizations — and not for lack of trying, nor for lack of solar systems to check out. Indeed, we know there are so many trillions of stars in the universe, many of which are circled by inhabitable planets. So where are all the aliens?
