That the Moon is intrinsically dark and shines by reflected light was assumed by most ancient astronomers (with some exceptions). Indeed, if the Moon were a better reflector it would shine much brighter. The intensity of the reflected light divided by the intensity of the incident light upon the Moon from the Sun (= "albedo") is only 0.07, so that 93% of the sunlight that strikes the Moon's surface is absorbed.
Half of the Moon is always illuminated by the Sun (except during a lunar eclipse), though the illuminated half is not always turned toward the Earth. The far side is not the same as the dark side.