The scariest are the ones we have not yet discovered. The universe is so freaking big, our tiny brains can't comprehend its size. We'll never truly know everything that's out there.

Could be an incredible number of alien civilizations, both hostile or peaceful, or it could be, there's nothing and we are the only ones. There are also gamma ray bursts, we wouldn't know, but one could be heading to us right now.

And so much more...