Vera Rubin, who died on Christmas Day aged 88, was a US astronomer whose work helped establish the unsuspected existence of dark matter, about which little is known, but which with the similarly mysterious dark energy is now thought to make up 95 per cent of the mass of the universe.
Although the theories that made her name gained acceptance in the 1980s, Rubin had made the observations that led to them 30 years earlier. Yet so surprising were they, and so junior was she, that then they were greeted with scepticism. Her gender was also an obstacle in that it had closed many prestigious universities to her, giving her less standing. In hindsight, however, she believed that had she followed a more illustrious route into academia her mind might have been less receptive to impossibilities.
It was as a postgraduate that in 1949 she made calculations which did not accord with the accepted rate of the expansion of the cosmos. She suggested that stars might not be moving ever outwards but that instead entire galaxies could be rotating around an unknown pole, like planets around a sun.
She had insufficient data to substantiate this notion and her paper on it was rejected by the leading astronomical journals. But by the mid-1960s, when she joined the Carnegie Institution, in Washington, new spectrometers enabled her to make better studies of galaxy rotation rates.
By the Newtonian laws of gravity, the further a star is from the centre of a galaxy the less quickly it should orbit, much as Mercury circles the nearby Sun in 88 days but Neptune takes 165 years. Yet Rubin established that the different regions of the Andromeda galaxy moved at the same speed.
Something other than gravity was holding the stars in check - something unseen which did not emit light.
Dark matter had been posited (and dismissed) as a concept in the 1930s but Rubin and her colleague Kent Ford were the first to show evidence for it. Astronomers now think atoms constitute a small fraction of the universe but little is certain about the nature of the rest. Defining it is one of science's great tasks.
She was born Vera Florence Cooper in Philadelphia on July 23, 1928. Her father, an electrical engineer, came from Lithuania and her mother from modern Moldova.
When Vera was 10, the family moved to Washington DC. She became fascinated by the stars watching them from her bedroom window and built her first telescope from a roll of linoleum tube.
After winning a scholarship to Vassar College to study Astronomy, she applied to Princeton to do a master's but was rejected, the university not admitting women candidates until 1975.
So she went to Cornell, where she was taught quantum physics by two Nobel Prize winners, Richard Feynman and Hans Bethe. There she met her husband Robert, a mathematical physician. After teaching while bringing up their children, she took a PhD at Georgetown University. One aspect of her thesis theorised that galaxies cluster together rather than being distributed at random. Many decades later this was also shown to be true.
Her husband died in 2008. She is survived by three sons and a daughter.