Hypothesis: The Milky Way galaxy is rotating around its center, with stars and gas in the outer regions moving at higher velocities than expected based on visible mass, indicating the presence of dark matter.
To test this hypothesis, astronomers could measure the rotation curves of the galaxy by observing the Doppler shifts of spectral lines from stars and gas at various distances from the galactic center, comparing the observed velocities to the predictions made by Newtonian dynamics based on the visible mass distribution.