The moon rotates on its axis proportionally with the rotation of the Earth, so we only ever see one side of it and the far side is in perpetual darkness.
In fact, that’s mostly true. The moon does indeed spin in proportion with our planet so that we can only ever see exactly the same side of it from Earth.
The Sun, however, lights up the bit we can’t see as often as the near side of the moon. During a solar eclipse in particular, the far side gets completely illuminated while the near side to Earth is plunged into darkness.
The inset image (right) is of a 68-kilometre (42-mile)-wide crater called Poinsot on the far side of the moon, clearly lit up by sunlight, taken by the MoonKAM system on the Ebb spacecraft on 15 March 2012. One of the reasons why the ‘dark side’ myth has been perpetuated maybe Pink Floyd’s famous album entitled The Dark Side Of The Moon released in 1973.