How dark is the dark side of the moon?
The dark side of the moon is the hemisphere without sunlight shining on it. On the moon's surface it would be nighttime. The darkest conditions would be when the Earth was not in the sky and available illumination would be from starlight only. Without any atmosphere, it would be very dark indeed.
No different from the front of the moon that we see, which varies during the lunar month. When we see a full moon, the back side is dark, because the front is facing the sun. On a new moon, when the moon is dark to us, the back side of the moon is as bright because the back side is facing the sun.