According to Space.com, scientists have three main theories about the formation of the moon. They are the giant impact hypothesis, the capture theory and the co-formation theory.
The scientific community places the most credence on the giant impact hypothesis. Scientist believe that a Mars-sized body called Theia crashed into the Earth, yielding rocky debris that eventually coalesced to form the moon. Many scientist base their support of this theory on the composition of the moon. It is made of lighter elements that suggest that is was created from light debris from Earth's crust and not heavier elements found nearer the Earth's core.
The capture theory states that the moon was formed in another part of the universe and was captured by Earth's gravity as it passed. This theory explains the differences in composition of the Earth and moon, but it is problematic for a couple of reasons. First, most captured satellites are not spherical like the moon, and they have irregular shapes. Second, captured bodies don't align as neatly with their host planets; Earth's moon aligns nearly perfectly with Earth's ecliptic, the sun's apparent motion as viewed from Earth.
Proponents of the co-formation theory believe Earth and its moon formed around the same time from similar material. However, because the moon is less dense than Earth, many scientists discount this theory.