in that thick dark/darkness

Once upon a starless midnight there was an owl who sat on the branch of an oak tree. Two ground moles tried to slip quietly by, unnoticed. "You!" said the owl. "Who?" they quavered, in fear and astonishment, for they could not believe it was possible for anyone to see them in that thick dark/darkness.

Hi,
Do both dark and darkness fit in the above and mean about the same? IF not, what are the differences between the two words? Thanks.