US Astronomer Vera Rubin, whose pioneering work on galaxy rotation rates led to the theory of dark matter, has died at the age of 88 on Sunday of natural causes. She was living in Princeton, New Jersey. In 1974, Rubin discovered that the stars at the edges of galaxies moved faster than expected. <br/><br/>Her studies earned her numerous honours, including being the second female astronomer to be elected to the US National Academy of Sciences. But many questioned why she was never awarded a Nobel Prize.