Vera Cooper Rubin was a pioneering astronomer, an admired role model and a passionate champion of female scientists. Her groundbreaking work confirmed the existence of dark matter and demonstrated that galaxies are embedded in dark-matter halos, which we now know contain most of the mass in the Universe.

Credit: Carnegie Institution of Washington

In the 1970s, Rubin showed that the speed at which stars orbit around the centres of spiral galaxies remains high even at the outskirts. This contradicts the Newtonian theory of gravitation, which predicts that the speeds of distant stars should fall off as the pull of gravity declines, just as the farthest planets in the Solar System orbit more slowly around the Sun than do closer ones. The discrepancy is striking — if Jupiter moved at the same rate as Earth, for instance, it would orbit the Sun every 5 years, rather than every 12.

The only plausible explanation for these galactic 'flat rotation curves' (named for their shape on a graph) was that the mass of the galaxies must extend invisibly beyond the most distant stars and gas clouds. That excess mass is known as dark matter. Its existence was first suggested in 1933 by astronomer Fritz Zwicky, who saw that galaxies in clusters moved more quickly than would be predicted from observable mass.

Dark matter, which neither emits nor absorbs light, makes up 85% of the mass-density of the Universe. We don't know what it is for sure, but most astrophysicists think that it must be a new kind of particle, different from the familiar baryons (such as protons and neutrons) in stars, planets and people. The nature of dark matter is one of science's great unsolved mysteries. The answer will change how we think about the Universe.

Rubin made her discovery by working closely with her colleague Kent Ford, who built the sophisticated optical spectrograph needed for such accurate measurements. In 1970, they published their first rotation curve for the nearby Andromeda galaxy (V. C. Rubin and K. Ford Astrophys. J. 159, 379; 1970). Instead of observing hydrogen gas in galactic disks at 21-centimetre radio wavelengths, as others had done, Rubin and Ford chose to use spectral signatures at optical wavelengths. This avoided the difficulties of interpreting gas dynamics that might affect other studies. Their confirmation in 1978 of flat rotation curves in a large sample of ten spiral galaxies confirmed the existence of dark matter (V. C. Rubin et al. Astrophys. J. Lett. 225, L107–L111; 1978).

Vera Rubin passed away on 25 December 2016, aged 88. She was born in Philadelphia, Pennsylvania, in 1928 and moved with her family to Washington DC when she was 10. Her fascination with the night sky was evident at an early age. She would peer out of her bedroom window and wonder what caused the stars to move across the sky, or why the Moon seemed to follow her as she rode in the back seat of her parents' car.

Her insatiable desire to understand the Universe led her to study at Vassar College in New York state, which she chose because the first professional female astronomer in the United States, Maria Mitchell, had taught there in 1865–88. Rubin graduated in 1948 as the only astronomy major in her class, and was married the same year. She earned her master's degree at Cornell University in Ithaca, New York, then moved with her husband to Washington DC, where she received her PhD from Georgetown University in 1954. Her PhD thesis demonstrated that galaxies are clumped rather than evenly distributed in space — a surprising and crucial finding, the importance of which was not recognized until many years later.

After teaching at Georgetown, Vera accepted a research position in 1965 at the Carnegie Institution's Department of Terrestrial Magnetism in Washington DC, where she remained for the rest of her career. There she conducted her groundbreaking research with Ford, and mentored generations of young astronomers in the investigation of galaxy dynamics and dark matter. She was deeply loved and admired by all who knew her.

In spite of the numerous obstacles she faced as a female scientist, Rubin triumphed. She was always cheerful, passionate and persistent. She had wanted to attend graduate school at Princeton University in New Jersey, but was denied because the university did not accept women at the time (Princeton awarded Rubin an honorary degree in 2005). She wanted to use the Hale telescope at the Palomar Observatory in California, but was denied because the facility did not allow women to do so until the 1960s. She paved a path for women not only by encouraging and inspiring them, but also by pressing for them to be hired for faculty positions, to be awarded honours and to be invited to speak at conferences. If too few women were listed as speakers, she would demand that organizers add more. As Vera liked to say, “Worldwide, half of all brains are in women.”

Vera's accomplishments have been recognized by numerous honours, including the US National Medal of Science, the Gold Medal of the Royal Astronomical Society in London, the Gruber Foundation Cosmology Prize, the James Craig Watson Medal of the US National Academy of Sciences and many honorary degrees. She was widely seen as deserving of a Nobel prize for her pivotal observations. Vera's legacy as the 'mother' of flat rotation curves and dark matter will be forever remembered, as will her role in mentoring and inspiring generations of scientists, male and female.