"it's so dark outside" => 밖은 굉장히 어두워요 / 밖은 아주 어두워요. The above sentences aren't likely to be heard very much in real life. If the person just realized it, for example, they will say 밖은 벌써 (굉장히) 어두워요 (It's already dark outside), which is probably said a lot more.

밖은 너무 어두워요.