There are varying theories on how the moon was created. Some scientists believe that it was created when a Mars-sized planet named Theia crashed into the Earth during the early years of the formation of the solar system.
The debris that resulted from this collision eventually came together to form the moon, which then became Earth's one and only natural satellite. However, some scientists don't quite believe the theory because the isotopes found on the moon are very much like those found on Earth. This suggests that the moon broke off from earth or was made from the same material.
Other scientists believe in a similar but more likely scenario where two protoplanets collided. The debris formed both the Earth and the moon.
Another theory is that the moon was captured by Earth's gravity, though some scientists believe that the isotopes are too alike for this to be true. George Darwin, son of Charles Darwin, believed that the Earth spun so rapidly at one point that it ejected a chunk of itself that became the moon. Still other theories posit that the Earth and moon simply evolved together out of the accretion disk that formed the solar system or that the moon was the result of a georeactor exploding. A georeactor is a uranium deposit where nuclear reactions occur naturally.