There’s been a recent spate of popular interest in the Great Filter theory, but I think it all misses an important point brought up in Robin Hanson’s original 1998 paper on the subject.
The Great Filter, remember, is the horror-genre-adaptation of Fermi’s Paradox. All of our calculations say that, in the infinite vastness of time and space, intelligent aliens should be very common. But we don’t see any of them. We haven’t seen their colossal astro-engineering projects in the night sky. We haven’t heard their messages through SETI. And most important, we haven’t been visited or colonized by them.
This is very strange. Consider that if humankind makes it another thousand years, we’ll probably have started to colonize other star systems. Those star systems will colonize other star systems and so on until we start expanding at nearly the speed of light, colonizing literally everything in sight. After a hundred thousand years or so we’ll have settled a big chunk of the galaxy, assuming we haven’t killed ourselves first or encountered someone else already living there.
But there should be alien civilizations that are a billion years old. Anything that could conceivably be colonized, they should have gotten to back when trilobytes still seemed like superadvanced mutants. But here we are, perfectly nice solar system, lots of any type of resources you could desire, and they’ve never visited. Why not?