Near to first quarter and through the last quarter phase we see only half of the side of the moon that is Earth facing. When we see a full moon the side of the moon we are looking at is facing the sun and, thus lit. At this time only could you say that the far side of the moon is dark. The moon rotates on its Axis once every 28 days which is also the time it takes to complete 1 revolution of Earth. We only ever see one side of the moon. Likewise the inhabitants of the far side of the moon would have know knowledge of planet Earth. They would never have seen it. Unless they go walkabout.