The leading theory for the creation of Earth's moon is that 4.5 billlion years ago a protoplanet about the same size as Mars called Theia collided with Earth, ejecting matter from both planets which came together to form the moon. When this happened, Earth's iron had already started sinking down to form the core, meaning Theia's impact wouldn't eject iron. Considering that we would expect the moon to not have iron, which is exactly what we see. Also, since the Earth is less dense nearer to the surface, we would expect the moon to have a lighter density than Earth if the Theia impact theory is correct. These expectations are a reality, since moon is only has 60% of the density Earth has. Lastly, the impact of a protoplanet wouldn't be suprising
