I have this underground bit, thats too bright. I want it to be completely dark. Any help with this? And no, moving whole map doesnt help a bit

You could add this kvp on your worldspawn:

ambient      0

I'm not sure if there will be light left but it should be very dark then. This way you would have to add lights everywhere where you want to see something even though the sun doesn't directly reach it.

I'd recommend playing with this setting a bit, maybe it'll do the trick.