While different parts of the moon are lit up at different times, we never see it all as it rotates with only one side facing the Earth. The Moon’s dark side remains hidden because the Moon rotates around its spin axis at the same speed as it orbits the Earth. This is called synchronous rotation or tidal locking. The Open University has an excellent short animated feature narrated by David Mitchell called The rotating moon, which explains it all in 60 seconds.
Nasa’s Lunar Reconnaissance Orbiter shows us the moon’s dark side
Using data obtained by the Lunar Reconnaissance Orbiter, NASA has created a detailed animation revealing what the dark side looks like. The Lunar Reconnaissance Orbiter was launched in 2009 to take 3D maps of the moon to help identify future landing sites for possible human and or robotic missions.
The side of the moon visible to Earth is covered in lunar maria, dark patches of cooled lava from ancient volcanoes. The dark side in comparison has less cooled lava and significantly more craters.
Watch the moon’s dark side spin in the video above.