Galaxies, Galactic Rotation, and the Hidden Mass

Hi, I'm Lumi. Let's look at a galaxy together. A spiral galaxy like ours, the Milky Way, has a bright central bulge, a flat disk of stars and gas spinning around the center, and a faint outer region called the halo. Most of the light comes from near the middle, where stars are packed tightly. Now think about gravity. In our Solar System, planets close to the Sun orbit fast, and far-out planets orbit slowly, because almost all the mass sits in the Sun at the center. So if a galaxy's mass were also mostly in its bright center, stars far out in the disk should orbit slowly too. That is the prediction. But when astronomers measured real galaxies, they found something surprising. Out past the bright center, stars do NOT slow down. Their orbital speed stays roughly flat, even far from the glowing core. A graph of speed versus distance, called a rotation curve, stays high instead of dropping. This only makes sense if lots of extra mass is spread far out, well beyond the visible stars. We cannot see this mass in any telescope, so we call it dark matter. It does not give off or block light, but its gravity holds fast-moving outer stars in their orbits. Flat rotation curves are the main piece of evidence that most of the matter in the universe is dark matter — outweighing visible matter by roughly five to one — along with gravitational lensing and other independent observations. If this feels strange, that's okay. Try the activity below to build the idea step by step.