Re: in the dark, in the darkness
"The dark" and "darkness" both mean "absence of light". The difference lies mainly in the kind of contexts in which each is likely to appear, and is therefore very difficult to summarise. (Though another member may be able to do so.)
But here are some characteristic examples:
1. We were left completely in the dark. [not "darkness"]
— metaphorical: = "...in ignorance".
2. I was sitting in the dark.
— perhaps more common with an "outdoors" reference.
3. I was sitting in darkness.
— perhaps more common if the darkness is within a covered place.
4. The darkness was almost tangible.
— "darkness" is presented as a substance; "the dark" might not quite be idiomatic here.
5. The dark of the moon.
— the dark time of the moon: i.e. when there is little moonlight, around the time of the new moon.
Not a professional ESL teacher.