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Giant-impact hypothesis
The leading explanation for the Moon's origin: about 4.5 billion years ago a large body — usually called Theia — struck the young Earth, and the Moon formed from the debris. Several theories still vie for dominance, but almost all share that impact; what is open is whether it was a single Mars-sized object or a series of objects.
NASA writes that several theories about the Moon's formation vie for dominance, but almost all share one point: near the time of the Solar System's formation, about 4.5 billion years ago, something crashed into the young Earth and flung enough molten and vaporised debris into space to create the Moon. Where the theories still differ is in the detail of that impact: a single object the size of Mars, or a series of objects.
That massive object was named Theia, after the mythological Greek Titan who was the mother of Selene, goddess of the Moon. The Apollo evidence all pointed to the Moon forming from a large impact: the type and composition of the samples showed the Moon had been molten during its formation and was covered with a deep ocean of magma for tens of millions to hundreds of millions of years — the environment that would follow an intensely energetic impact.
Open questions remain that any formation theory has to account for, among them the Moon's unusually low iron content and the differences between its near and far sides.