Marginal difference in meaning, most relevant difference is in tone/connotation: "the darkness" = midnight, vampires, no clouds or moon at night, utter darkness (complete darkness) strong wind and suddenly wooosh the candle went out, wolf howl... "dark" = not light

I think you want the difference between "in the dark" and "in darkness" "In the dark" is an idiom that means that you are not informed about something, that you don't know what is happening. "When asked about his friend's divorce, Tom said he was totally in the dark about it." "After the meeting, we were still in the dark. We had no idea what was happening." "In darkness" just means literally, physically without light. "When the power failed, we were left in darkness."