
Aleksey S. answered 04/17/19
Russian Tutor for everyone
This phrase is an adverb that has already shaped itself into this set form that it would be hard to find logic in why the first part got that ending. Most probably such phrases as давным-давно, полным-полно, светлым-светло, темным-темно, etc. had formed long before there was a clear distinction beween the modern cases of Russian adjectives. Even though the first part of the adverb looks similar to the dative case of adjective тёмный - тёмным, it would not be logical to explain the formation of it through that case.