There are many theories regarding the formation of Moon, the Earth’s only natural satellite. Sister theory states that the Moon was formed along with Earth during the formation of our Solar System. Another theory states that it is a captured Moon like most of the moons of gas giants. However, these theories could not explain how it was actually formed, with each having its own flaws.
Another theory called Giant Impact Hypothesis or Theia Impact theory states that the Moon was formed after a planet called Theia, which existed in the early solar system collided with the Early Earth, when it was forming. Models suggest that it was an Earth Trojan about the size of Mars; with a diameter of about 6,102 Km. It orbited the Sun-Earth system at L4 or L5 point. Computer simulations suggest that when it collided with early Earth at almost 4.5 billion years ago, it was traveling at no faster than 4km/sec. In 2019, all the evidence suggested Theia was formed in the outer solar system rather than inner solar system.
After slamming into Earth, the outer rocky shells of both Earth and Theia were blasted into a disk of debris around our planet. From this disk, the Moon coalesced. This theory also explains why Earth has a bigger core than would be expected for a body of its size because Theia’s core and Mantle mixed with Earth’s core and mantle.
This theory is the most trusted theory because the lunar rocks brought by the Apollo program astronauts were found to have a similar composition to that of Earth’s crust. So, more likely they were removed from Earth in some violent event.