There are some who say that the universe can bring about life as a common event and that us and our life-serving environment here on Earth is nothing unique. If that's true, then we ought to expect life to develop everywhere and even intelligent life to at least be somewhat common and we ought to expect to see some signs of advanced technological species operating in our galaxy.
But where are they?!
Through calculating an exponential expansion from one star system to the next at a reasonable pace, it is not unreasonable to find that a single species could colonize the entire galaxy in as little time as ten million years. Through such activities we should expect to see obvious signs of artificial interaction with bodies in space. Mined stars, gathered nebulae, building huge constructs, all sorts of high engineering projects with the goal of using the natural resources for a species' own purposes. But we see none of that. The galaxy is virgin. We don't even hear any radio signals, despite SETI being active for decades already.
Surely the galaxy has been around for much longer than ten million years, so there should be little time constraint involved. Then where are the aliens?
This is what is known as the Fermi Paradox.
Some explanations for this apparent incongruity have been proposed. Perhaps the aliens are intentionally hiding themselves. Perhaps none survive to a point where they are capable of interstellar travel. Perhaps the aliens are simply not that aggressive and feel no urge to reach for the stars.
Or perhaps life is not so prevalent in the universe and we are indeed highly unique - and alone.