For nearby spirals, their rotation curves help measure both light and dark matter.

Question

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The rotation curve of a spiral galaxy can be used to measure both the distribution of visible matter (like stars) and the distribution of invisible dark matter. By measuring the rotation of stars and gas at different distances from the galactic center, astronomers can determine the total mass of the galaxy at those distances. This allows them to map the distribution of dark matter, which does not emit or absorb light but still exerts a gravitational force on the stars and gas in the galaxy. Yes, the rotation curves of nearby spirals can be used to measure both the visible matter (gas and stars) and the invisible, or dark, matter that must be present to explain the observed rotation speeds.

For Nearby Spirals, Their Rotation Curves Help Measure Both Light