You’ve probably heard that our Sun is located in the Milky Way galaxy. But what is a galaxy anyway? The simple answer is that a galaxy is a collection of stars held together by their mutual galaxy. In other words, all the stars in a galaxy are kept together by the gravity of all the other stars (as well as the invisible, mysterious dark matter).
But the stars we can see are just a tiny fraction of the complete galaxy. It’s also surrounded by a vast halo of dark matter. This material is invisible, and doesn’t interact with regular matter or give off any kind of radiation that we can detect. But astronomers can measure its effects because it does exert a gravitational force on other matter. In fact, the Milky Way is made up of mostly dark matter. The stars account for about 580 billion solar masses, and the dark matter could be another 6 trillion solar masses.