the moon makes on spin on its axix for 1 orbit around the Earth, that is why it only shows one face to observers on the Earth
take a ball and mark a dot its equator and then move it around another ball like an orbit
notice that in order for the dot on the ball to face the inner ball, it has to make one spin on its axis for on orbit around the other ball
now set up a light coming in from one side of the ball with the dot orbiting the other ball
notice that the lighting on the ball with the dot exhibits the lighting of the phases of the moon, but the back side of the ball with the dot also exhibits the phases as well, just at opposite times
so the "dark side" of the moon is not dark at all… it also has a day and a night