During its early stages of formation, a catastrophic collision with Earth by a large body is thought to have ripped part of the Earth away to form:

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Quizplus · Accepted Answer

The prevailing theory, known as the giant-impact hypothesis, suggests that the Moon formed from the debris left over after a collision between Earth and a Mars-sized body, often referred to as Theia, early in the Solar System's history.

During Its Early Stages of Formation, a Catastrophic Collision with Earth