'-_-' is an internet 'type up' emoticon that has a few meanings.
People usually say that '-_-' represents a sign of disgust but it can also be a face that represents tire over someone who you find cringy or annoying online.
May also represent extreme suspicion over a particular subject. '-_-'s little brother is '-.-' which could shows even more suspicion.
Primary usage
P1: Lol did you see Brad's profile pic? He just dropped 15 pounds!
P2: Slim, eh?
P1: Uh, yeah?
P2: Lol P1: What?
P2: Have you ever thought about what gets destroyed in this universe, and how it gets destroyed?
P1: Uhhhhhh
P2: You're telling me he's so thin that he doesn't exist, creating a vacuum in space... our world is in danger...
P1: Wow dude, just WOOOOW... -_-