https://phys.org/news/2019-01-side-moon-dark.html

"The other side sees the sun sometimes. The other side is not dark, it's just far," said Harvard astronomer Avi Loeb. "It's a mistake."

The moon is what scientists call "tidally locked " which means the same side always faces us, while another side always faces away, Loeb said. When Earth views a darkened new moon, the far side is lit. When there's a full moon in our sky, the far side is dark.