ASTRONOMERS have discovered a galaxy without any stars. It behaves like a galaxy, but however hard they strain their eyes, the astronomers can detect not a twinkle. They conclude, in Astrophysical Journal, that the galaxy consists entirely of dark matter.

The first such dark galaxy to be detected, VIRGOHI21 was found using radio telescopes.

A dark galaxy contains a large amount of matter too thinly spread to create the conditions for star formation. Its presence can be inferred from the behaviour of surrounding matter and by its rate of rotation.

Just what form that matter takes is controversial. Some of it may be brown dwarfs, stars too small to generate the nuclear reactions that make stars shine, subatomic particles, black holes or neutron stars.

A team from Britain, France, Italy and Australia saw the dark galaxy in 2000 from the University of Manchester Lovell telescope in Cheshire, but it has taken almost five years to rule out other explanations.

Mike Disney, of the Cardiff University-led team, said: “As Sherlock Holmes famously said, ‘When you have eliminated the impossible, whatever is left — however improbable — must be the truth.’ ”

The team searched for dark galaxies by studying the distribution of hydrogen atoms. In the Virgo cluster of galaxies, about 50 million light years away, they found a mass of hydrogen atoms a hundred million times the mass of the Sun.

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