Dark Matter Halo

The visible disk of the Milky Way is embedded in a much larger, invisible dark matter halo that reaches roughly 300,000 light-years from the center. This halo holds about 10 times more mass than all the visible stars, gas, and dust combined. Without it, the galaxy would fly apart.

The halo is roughly spherical, unlike the flat disk of stars, and was the first structure to form. It provided the gravitational scaffolding that pulled in normal matter and let the galaxy assemble over billions of years.