Finding darkness in the light

As a galaxy rotates, the stars move around its core. If it’s edge-on to us, then on one side, the galaxy rotates “towards” us, while the other side rotates “away from” us. The faster the galaxy rotates, the faster the “towards” and “away” motions are. If the rotations are fast enough and your instruments are good enough, you can actually measure this effect. This was the incredible possibility that Vera Rubin began investigating. Thanks to advances in spectroscopy — the capability of breaking light up into individual wavelengths, detecting emission and absorption lines — Vera Rubin and Kent Ford started taking measurements of nearby galaxies in an attempt to measure their rotation speeds.

via https://medium.com/starts-with-a-bang/finding-darkness-in-the-light–976a613bdaca