The origin of the Moon is still under scientific discussion.
The most frequent explanation involves an early catastrophic impact, when the Solar System was still in formation, about 4,5 billion years.
This “traditional theory” implies that a Mars-sized object called Theia, just like the Greek goddess -mother of Selene, goddess of the Moon- collided with young Earth (or at least 90 % of it), and this caused a considerable impact that melted and vaporized Theia, creating a significant damage on Earth. Part of the material coming from Theia stayed on Earth, and another part was lost to space and were accumulated, only by gravity, to create what we know today as the Moon. This theory offers reasonable explanations to understand some of the properties we see both in our planet and in the Moon, like its distance, circular orbit, and the fact that its rotation is in sync with its orbit rotation (and that is why it shows us the same face). However, considering the new analysis of lunar rocks brought by the Apollo mission, which shows that these rocks have an Earth-alike composition and that Theia having the same composition of former Earth is lower to 1 %, other theories have come up. One of them suggests that, if there were two planets, five times bigger than Mars each, would form a disk when they collide, so the formation of the Earth and Moon can be explained, and, in this case, both would share the same composition indeed. However, the last word has not been said yet, so it is essential to keep exploring our natural satellite.
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