어둡다 is the infinitive "to be dark", but yeah your formal version is right
Should 'gloomy' be accepted? I learned dark / gloomy as equal translations but was wrong.
I spelt restaurant wrong by accidentally writing restraunt instead and it said I used the wrong word. WHAT? can someone explain
Well, restraunt is wrong. It's that simple.
Sometimes, Duolingo accepts typos, but those are obiously not hard-coded but algorithmically get detected with some level of tolerance with respect to the correct word. Your typo didn't fit pass this algorithmic test.