Astronomer Vera Rubin, whose pioneering work on galaxy rotation rates led to the theory of Dark Matter, has died at the age of 88, her son says.
In 1974, Rubin discovered that the stars at the edges of galaxies moved faster than expected.
Gravity calculations using only visible matter in galaxies showed that the outer stars should have been moving more slowly.
To reconcile her observations with the law of gravity, scientists proposed there was matter we cannot see and called it dark matter.
Dark matter is an unidentified type of matter comprising approximately 27% of the mass and energy in the observable universe.