The article is used when you speak of the darkness of the night. It is specifying night time (usually). I looked out into darkness... Here darkness could be unexpected. So, it's daylight and yet when you look outside, there is darkness. Or the lights are unexpectedly off. Or something like that.

Good question. You can say this both ways: "I looked out into the darkness" (Here, "the darkness" is considered a thing, so the article 'the' makes sense.) or "I looked out into darkness" (this sounds poetic, so maybe it is not good for every situation.) Good luck!