Sure its correct!

Oh, I see... Thank you, Chris

It depends on what you're trying to say. "It's dark outside." would mean that it's night time. "There is darkness outside." would mean that there is some kind of evil outside.

thanks, LEEZA

I am not sure if it is correct or not, as I am not a grammar whiz. That said I would not write it that way, I would have written ' It is dark outside ' Or for a fancier version, perhaps ' darkness is descending '