Vera Rubin’s spectrograph of the galaxy NGC 7541. Each of the spectral lines emitted by the galaxy makes an image of the galaxy which are spread from left to right depending on their wavelength. (Five images can be clearly seen.) The lines are red shifted by an amount that depends on the velocity of the source relative to us. The galaxy is moving away from us due to the expansion of the Universe, but the galaxy is rotating around its center, so one side is rotating away from us and one side is rotating towards us. This means the two halves of the galaxy have slightly different red shifts as is apparent down the center line. What is remarkable is that the red shift stays constant as we move away from the center. If the only mass in the galaxy was from stars then the points farther out on either side would have more similar red shifts than points closer in. This was the first solid evidence for dark matter.
For more information see Philip and Phylis Morrison “The Ring of Truth” Vintage Books, New York 1989.