Since the galaxy is billions of years old, he expressed what became known as Fermi's Paradox by asking a simple question, "Where is everybody?"
The belief that the universe must be teeming with life, and much of it must be highly intelligent, has come to be known as the "theory of mediocrity" of the Earth and its inhabitants. It is the idea that there is nothing exceptional about our planet, its life forms and especially homo sapiens, that we are all merely average examples of that found across the galaxy.
That belief (which has no actual scientific data to back it up) gave rise to the still-ongoing SETI project (Search for Extraterrestial Intelligence). However, SETI continues to come up empty and some of its researchers are starting, perhaps, to ponder a shocking possibility - that homo sapiens is one of the first, maybe even the only, highly-intelligent life form to have developed in the galaxy.
Science writer Mark Thompson explains that even a 14-billion-year-old universe may not be old enough to result in planets teeming with life, especially intelligent life.
It seems that the evolution of stars precluded the formation of rocky planets much before the appearance of Population I stars. If that is the case, and adding a generous margin for error, it looks like the first planets like Earth would have formed no earlier than 8 billion years ago.
If that is true, then it may well be that we are not necessarily the first life, but perhaps amongst the first intelligent life (as we know it) to evolve.Furthermore, there is no teleology in evolution theory. No outcome is inevitable, there are no such things as "higher" life forms. There is only survival, or not. Hence, technological, inventive beings are not a rational or inevitable development of evolution at all. There is no "rational" development of life in the first place and no evolutionary outcome is inevitable in any way.
Harvard biologist Ernst Mayr has pointed out that since life first appeared on Earth, there have been an estimated 50 billion species. And yet only one, us, has developed high intelligence. Mayr says that such intelligence does not obviously offer a species survival advantage and hence may be so rare that homo sapiens may be a "one off" in the universe.
In 2011 I put together a slide presentation for the topic to discuss Fermi's Paradox at church. You will probably be surprised at the conclusion. Here 'tis:
But for argument's sake, let us stipulate that in the known universe there are at least 200 billion species that are at least as intelligent as homo sapiens. Sound like a lot? It averages only one per galaxy.