Translation:Los inviernos aquí son muy oscuros.
did you leave out aquí?
All the other examples use estar with oscuro - why not here?
I'll go out on a limb and guess that this is a sweeping generalization about all winters in that area over all time and not about a particular day of a particular winter that might be different from the next day.
I used aquí which was accepted but why not acá?
Why have you said I should of put aca?
Latin American way of saying aquí.
Why does this sentence use Los inviernos instead of Inviernos?
In Spanish, nouns cannot stand on their own as they do in English. You need an article (a, an, some, the).