The light spots on the moon are called lunar highlands. The darker areas of the moon are called maria, and they are lower in altitude than the lighter spots. The highlands lie several kilometers above the maria.
The highlands consist of rocks with high aluminum content; therefore, the highlands are lighter in color and less dense than the maria. Because the moon always shows the same face to the Earth, scientists focused on this side and saw that it contained a relatively equal distribution of maria and highlands. When the far side of the moon was explored, scientists found that the far side consisted mainly of highlands.