Something that a lot of people presume when talking about alien life, I've noticed, is that they've somehow solved the lightspeed barrier. We have a lot of ideas about how it might be done, but we've still not proven that any of them are actually worthy of further investigation, and so we're just sort of flailing out of desperate hope for such an ability. There's nothing saying that even a hyper-intelligent apex civilization is able to travel or communicate faster than the speed of light, so it compels me to wonder if that might have something to do with the emptiness of the universe we live in where our nearest dead neighbor would take centuries to reach with current technology and the nearest maybe living neighbor would take millenia. It's just so big that to come in contact with another species is just as miraculous as surviving long enough to catch them.
I also get the impression that there are a couple of major filters — namely biopoiesis and sapience. As Urban mentioned, the former has never been recreated in a lab and the latter has only one surviving victor, and furthermore evolution doesn't guide everything toward intelligence. I reckon that sapience in humans comes out of a positive feedback loop facilitated by sexuality and doubled by culture (we generally don't seek partners with lesser intelligence and we have consistently raised our children in a more advanced society than that of our childhoods since the era of hunter-gatherers). Biopoiesis is just something that comes out of dumb chance as far as I know, so I guess that just doesn't happen very often in the first place.
So, what about that one species that was spawned in a boiling chemical bath, miraculously took their world stage by the reigns and rode into the Type III sunset with their automated replicants or something? That's a good one, I think. Maybe we're it; maybe we're in that last sliver where they just haven't gotten, yet; maybe they skipped our planet on purpose because of the Prime Directive; maybe they just didn't bother because why would they bother colonizing a galaxy when they can just spawn a miniature universe and tap it for its potentially infinite energy — maybe that's what dark matter is: Missing matter that was taken from us by our universe's host!
Wow, that really is a good one. I never thought about a real species that just colonizes and colonizes and doesn't stop. It always just seemed so primitive for a species that could do things with far more ease otherwise.