In order to unravel the lingering mystery of the formation of Earth's everlasting companion, NASA launched the twin Gravity Recovery and Interior Laboratory (GRAIL) satellites in 2011, which have done a terrific job mapping out the Moon's surface gravity in unprecedented detail.
This is the result -- the moon's GRAIL gravity map. The areas that have slightly less gravity are pictured in blue, while the regions that have stronger gravity can be seen in red. Furthermore, analysis of GRAIL data has revealed that that the moon has a shallow crust, which runs approximately 40 kilometers deep, with its overall composition very similar to that of the Earth.
The most prevalent theory of the moon's formation, which all of the evidence is heavily in favor of, is that a large Mars-sized object (called "Thea") impacted Earth, some 4.5 billion years ago -- launching a debris cloud around Earth, which eventually coalesced into our moon.