Horace Babcock, measuring the rotation rate of the Andromeda Galaxy, obtained the unexpected result that the rotation of the outer parts of the disk was much faster than expected from Newton’s laws of universal gravitation. These results, in the 1970s and 1980s, were generalized to all galaxies of this type. Furthermore, the discrepancy increased from the center of the galaxy to the periphery. This evidence for the presence of dark matter.
