the state or quality of being dark; darkness or gloom (the opposite of brightness).
部屋の暗さが気になるので電気をつけた。
The room's darkness bothered me, so I turned on the light.
夜の暗さが街に静けさをもたらした。
The darkness of the night brought quiet to the town.
彼の表情の暗さが心配だ。
I'm worried about the gloominess in his expression.