Star gazers discover first 'dark' galaxy

British astronomers have discovered a cosmic phenomenon that appears to be an invisible galaxy made almost entirely of dark matter, the first ever detected.

The dark galaxy, which contains a large amount of mass that rotates like a galaxy but contains no stars, was first seen with the University of Manchester's Lovell Telescope in Cheshire, and the sighting was confirmed with the Arecibo telescope in Puerto Rico.

The unknown material that is thought to hold these galaxies together is known as 'dark matter', about which scientists still know very little. An image of the area in which the phenomenon was found can be seen here.

Dr Jon Davies, one of the team of astronomers from Cardiff University, said: "The universe has all sorts of secrets still to reveal to us, but this shows that we are beginning to understand how to look at it in the right way. It's a really exciting discovery."

In a bid to minimise so-called light pollution, an international team from the UK, France, Italy and Australia has been searching for dark galaxies using radio waves.

They have been studying the distribution of hydrogen atoms throughout the universe. Hydrogen gas releases radiation that can be detected at radio wavelengths.

In the Virgo cluster of galaxies they found a mass of hydrogen atoms a hundred million times the mass of the Sun. The Virgo cluster is a large group of galaxies about 50 million light years away.

Dr Robert Minchin, from Cardiff University, one of the UK astronomers who discovered the galaxy, named VIRGOHI21, said: "From the speed it is spinning, we realised that VIRGOHI21 was a thousand times more massive than could be accounted for by the observed hydrogen atoms alone.

"If it were an ordinary galaxy, then it should be quite bright and would be visible with a good amateur telescope."

Similar objects that have previously been discovered have since turned out to contain stars when studied with high-powered optical telescopes. Others have been found to be the remnants of two galaxies colliding.

However, when the scientists studied the area in question using the Isaac Newton Telescope in La Palma, they found no visible trace of any stars, and no nearby galaxies that would suggest a collision.

Professor Mike Disney, a member of the UK team of astromers, said: "As Sherlock Holmes famously said: 'When you have eliminated the impossible, whatever is left - however improbable - must be the truth.'"

Astronomers have been measuring the way in which stars and galaxies move for many years. These measurements indicate that there must be far more matter in the universe than can be accounted for by the visible light we see.

The presence of dark matter and this lack of galaxies have led some astronomers to predict that there must be unseen 'dark' galaxies hidden in the universe.