Just started playing this mod, and all of my systems have a +2 unhappiness "AAAAARGH" modifier. What causes this and how can I get rid of it?

I think that is part of the trait for one of the leaders. I am not sure though.

Yeah, that or your difficulty level. Any of the really obscure ones like 'We're just happy to be here' or 'Arghhhhh' are global modifiers applied by the difficulty level or a civ-wide trait.

Nah, difficulty level is expressed as how many free happy faces you get, not as added unhappy faces

you're playing with New Earth, and they start with 2 pop. instead of one, but they have +2:mad: / +2:yuck: per solar system; what you are seeing is the :mad: penalty
if you want to get rid of it, play with another civilization