Translation:Isn't it dark?
I was also confused, but thinking it over I think this is a justifiable translation (I'd say "it isn't dark?" is also correct).
Let's say if you genuinely questioned whether or not it was dark, then both "isn't it dark?" and "it isn't dark?" have the same meaning given the right tone of voice. Thinking less "wow, isn't it dark" and more "hmm...isn't it dark?" helps me.
I'm guessing it's because it has か at the end, so it wants the question quality to come through without punctuation (which Duolingo typically doesn't require). "It isn't dark" requires the question mark, whereas "Isn't it dark" stays a question based on grammar alone if you drop the question mark.
The Japanese way of asking and answering this kind of question is a bit counter intuitive for English speakers, which is why the translation might seem strange to you. This is a question that is expecting a yes or no answer. I don't think that "it isn't dark?" achieves that expectation for a yes or no answer, because it sounds like a rhetorical question or like you're disagreeing with someone who said that it's dark.