Something is filtering out almost every civilization before it can spread

Economist Robin Hanson's 'Great Filter' reframes the Fermi paradox: given how many steps separate dead matter from a civilization visibly colonizing the galaxy, and given that we observe none, at least one of those steps must be extraordinarily improbable. Whether that filter sits behind us, like life's origin, or ahead of us, like collapse, changes how rare, or doomed, we should think we are.

— Robin Hanson, The Great Filter - Are We Almost Past It? — 1998