In the last 20 years, we've discovered nearly 4500 planets outside of our solar system. And they're not rare. Everywhere we look we see planets; all signs indicate that there are billions of planets strewn throughout the milky way galaxy, and countless more in the other galaxies scattered across the universe.
When we consider that the universe is 13.8 billion years old, (of which modern man has only been around for a tiny fraction), and there are countless more opportunities for life to exist outside our solar system, the idea that we are alone in the universe is laughable.
And yet, the signs that we think would identify life outside this world are notoriously absent. No signals, no large scale construction projects (like a Dyson sphere, Matrioshka Brain, or Death Star), no inconsistencies in the electromagnetic spectrum across the vastness of space. We scan the stars with our telescopes, and it certainly seems like we're alone.
This juxtaposition of facts is what is known as the Fermi Paradox. Put in the form of a question, it would ask: "If the circumstances favorable to life are so common, then why don't we see any signs of life?"
One interesting answer involves a concept known as the Great Filter. The Great Filter is a hypothetical event that occurs in the development cycle of a form of intelligent life. Here's how it works.
As far as intelligent species goes, we only have one data point (humans), so we need to make some assumptions. Let's assume that any other life form in the universe would develop in a relatively similar manner to us. For example, we could assume that at some stage of development they had to transition for single-cell to multi-cellular organisms. We'd assume they need to reach other developmental landmarks as well, like sexual reproduction (for genetic diversity), transitioning from a foraging to agricultural lifestyle, the discovery of nuclear technology, and other landmarks that we have yet to reach ourselves (like synthesizing life, perhaps).
Assuming this is true, it's possible that one or more of these developmental landmarks is incredibly rare. For example, it's possible that the universe is teeming with single-celled life, but it's exceptionally rare for it to develop into multi-celled life. This landmark would act as a filter, one that Earth-based life was fortunate to pass through. If other lifeforms struggle to make that leap, it would explain the apparent absence of advanced lifeforms in the universe.
The foreboding flipside of this idea is that with our single data point, we do not know which developmental landmark (or combination of landmarks) is the filter. Maybe it's rare for species to develop enough technology to defend against asteroids before an astroid wipes them out. Maybe it's rare that they discover nuclear technology without destroying themselves. And maybe the Great Filter is some event that humans haven't yet passed through… something we probably won't survive through.
We want to believe we're special. That we're the species that made it through all the filters, and that we're destined to rule the universe. But is that likely, or is it just an instance of fundamental attribution error that operates on the macro level rather than on the individual? Since we can't know for sure (again, only one data point), it's best to not be over-confident. As Nick Bostrom says in this fantastic article "Omens":
"The more advanced life we find, the worse the implications. If Curiosity spots a vertebrate fossil embedded in Martian rock, it would mean that a Cambrian explosion occurred twice in the same solar system. It would give us reason to suspect that nature is very good at knitting atoms into complex animal life, but very bad at nurturing star-hopping civilisations. It would make it less likely that humans have already slipped through the trap whose jaws keep our skies lifeless. It would be an omen."