Translation:This room is not bright.
"This room is not light" is an accepted answer. In my American dialect at least, that would only refer to the "weight" of the room (however one would measure that) so wouldn't make sense here (or much of anywhere).
However, I wonder if this answer is getting at something not necessarily well expressed by "is not bright." What does it mean? This room isn't well lit? This room doesn't have any lights? The lights in this room are off?