
When Vera Rubin measured the spin of galaxies, she found their outer stars moving so fast that visible matter alone could not hold them in place — one of the clearest early signs that most of every galaxy is made of something nobody has ever seen.
Beginning with the Andromeda galaxy in the late 1960s, the astronomer Vera Rubin and her colleague Kent Ford measured how fast stars and gas clouds orbit at different distances from a galaxy's centre.










