This is the 8726th most frequent Japanese word.
暗
"暗" means "dark" or "gloomy."
Here, '暗い' (kurai) means dark, and it is used to describe the room's lighting situation.
彼は暗い部屋で勉強している。
He is studying in a dark room.
In this sentence, '暗い' is metaphorically used to describe the future as pessimistic or discouraging.
将来が暗いと感じる瞬間もある。
There are moments when I feel the future is bleak.
Here, '暗' appears in '暗算' (anzan), where it conveys the idea of 'mental' as in done internally, not overtly.