Don't open your eyes. Nobody knows what started it, or why it started, and nobody even knows what the threat actually is. That is because anyone who actually sees it does not survive long enough to tell anyone else. The only guaranteed way to survive outside in Josh Malerman's apocalyptic setting is to be sure that you can't see a single thing. You must take drastic steps to ensure you won't accidentally see anything outside and any mistake will end your life. Needless to say, that makes things a little difficult for everyone because you can't stay inside forever...