I'm not worried about aliens. I think the chances of spacefaring life evolving on a given planet are so small that all aliens are outside our light cone. If aliens did exist in our light cone, we would see some evidence of them shaping the universe into structures they value. From what we can tell so far, all those stars are shining on lifeless planets. If we saw infrared leakage from Dyson spheres everywhere, or radio/microwave/whatever signals broadcast for primitive beings, or anything else like that, I would assign much higher probability to aliens existing.

Actually, if aliens were around, we would probably never have existed. Our solar system is made up of resources that can be used to make whatever it is aliens want to make. The fact that the earth has sat around for 4.5 billion years without being consumed by Von Neumann machines is pretty good evidence against aliens.

On the flip side, if I discovered aliens I would destroy them immediately. Their values would most likely be completely different from ours. The risk to humanity would too great to allow them to exist freely.

Edit: Tangentially-related short story: http://baencd.thefifthimperium.com/13-TheBalticWarCD/TheBalt...