From an article on CNET by Michelle Starr, February 12, 2015; visualizations by Ernie Wright:
As the Moon orbits the Earth, we only ever see the one side. This is because the moon is tidally locked – a single rotation of its axis takes the same amount of time as a single orbit around the Earth, so that the same side is always facing the Earth. Using its Lunar Reconnaissance Orbiter, NASA has collated data to reveal what the other side of the Moon looks like (see Section 6.2a, p. 127 and Figure 6-18, p. 133).
As the Moon goes through its phases, we see it darken and lighten as viewed from Earth. Those phases are the opposite of what the far side of the Moon experiences: when we have a Full Moon, the far side is new; when we have a New Moon, the far side is full. This means that the LRO can observe the far side of the Moon in pretty good detail when it is illuminated by the Sun.
In the years since it launched in 2009, the LRO has sent back hundreds of terabytes of data about the Moon’s far side. What it has found is that the far side of the Moon is quite different from the side we see.