Earth has a tilted angle regards to the solar plane, but moon's axis of rotation is almost perpendicular to this plane. This means that some regions at the poles of the moon never receive direct light from the sun. Add to this the fact that moon has no atmosphere to scatter the sun light. Some of these regions have remain in the shadow for 2 billion years!
It was only recently that these areas received some sort of direct light (other than the faint light received by the stars), and that is the laser beam from the LRO, NASA's Lunar Reconnaissance Orbiter. Watch this video to learn more about LRO and its sophisticated measuring instruments.