Theory says that the moon formed from the dust produced by a collision between earth and another large body.

Giant Impact Hypothesis is the theory that says that the moon formed from the dust produced by a collision between Earth and another large body.

Earth was formed from the leftover cloud of dust and gas orbiting around the sun. During the early solar system, large bodies kept on crashing with each other and many bodies were created from dust and gas but never achieved its planetary status.

According to the Giant Impact Hypothesis, one of this incomplete body called Theia crashed into Earth. The vaporized chunks of both Earth and Theia were flung into space and was bound together by gravity, thus creating the Moon. 

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