There is no such thing as the dark side of the Moon, or at least there is no area that is always dark. The Moon has a side that always faces Earth. When we see a full moon, the far side is dark, but when we have a new moon, the far side is fully illuminated by the sun. In other words, one lunar day is equal to approximately one Earth month. Supposing that part of a crater was always in the dark, there might be a very thin atmosphere, but not much since the Moon’s gravity is fairly weak.