The most comprehensive and widely held theory of how the moon formed is called the "giant impact hypothesis." That hypothesis shows that about 150 million years after the solar system formed, a roughly Mars-sized planet named Theia collided with Earth. Though the timeline is hotly debated in the scientific community, we know that this collision melted Theia and some of Earth, and that molten rock orbited around Earth until it coalesced into the moon.
But now a new study, though not contradicting the giant impact hypothesis, is suggesting a different timeline, and an older moon.
New research from scientists at the University of Cologne's Institute of Geology and Mineralogy suggests that the moon is older than the giant impact hypothesis says it is. Their research is based on chemical analyses of Apollo lunar samples and it shows that the moon formed only 50 million years after the solar system, rather than 150 million years. This ages the moon by 100 million years.