The Universe should be full of alien civilizations. Why can’t we find any?
I'd recommend you read the complete article. It's a quite interesting take on Fermi, UFOs and Aliens.
Here are a few excerpts...
“Where is everybody!?”
It comes down to a game of numbers. Intelligent life doesn’t have to be outright impossible. In fact, it can’t be—we’re here, and we’ve already begun taking our first steps into space. It just has to be so deeply rare that we shouldn’t expect to see any evidence for it elsewhere, despite decades of searching. So the game is to start with the plausible assumption of abundant life, then find a way to whittle that down to as close to zero as we can get it.
So where is the Filter? Is it early on, with the development of life itself? Is it somewhere in the middle, on the long march to intelligence? Is it at the end, when going from simple orbital jaunts leads to lengthy interstellar excursions?
In fact, humanity probably represents the Earth’s last shot. In just a few hundred million years, the Sun will grow too hot. Our oceans will boil, and we will turn into another Venus. So while life got started in Earth’s first chapter, intelligent life didn’t appear until its last.
So maybe that’s it. That’s the Great Filter: achieving intelligence. In that case, woohoo! We made it! Pop the champagne. We’re one of the extremely rare, lucky species that survived the Filter, and we have nothing but the stars in our future.
Or not. We really don’t know. We only have evidence for life on one planet. If we ever see signs of microbes in the dust of Mars or buried under the ice sheets of the outer moons, that might be a hopeful sign that we’ve made it through—that life is common but intelligence is not.