Gravity

Gravity is a force that makes things attract other things. Everything in the Universe is governed by gravity, including The Milky Way, The Solar System, the Earth, the Moon and all the other stuff that's out there. Unless the stuff has no mass, that is. Generally, it's noticeable that a heavy thing will attract a less heavy thing, but the less heavy thing  is also attracting the more heavy thing, so it all gets a bit complicated.