Vera Rubin is shown here in 1974, analyzing data from different portions of a galaxy to ascertain its rotational properties. The discovery that the effects of gravity did not trace out the same path that the starlight does was one of the most important discoveries of the 20th century, and brought dark matter into the mainstream of science from the fringes, where it had languished for most of the 20th century. Her work changed our conception of the Universe forever.