A picture is emerging that there is a lot of dark matter in the Universe and that most galaxies possess a great deal of it

Dr Neil Trentham

Astronomers may ultimately discover that dark galaxies outnumber the familiar visible kind by perhaps as many as 100 to one, according to three scientists from the University of Cambridge.

There is already considerable evidence that ordinary bright galaxies contain large amounts of dark matter - often 10 times more than the mass of all their stars put together.

Physicists have worked out that dark matter must exist to account for the observed movements of stars under the gravitational influence of galaxies. In some galaxies, there are so few stars that without the gravity of unseen matter to hold them together, they would have scattered into space long ago.

Difficult to detect

Dr Neil Trentham, one of the scientists predicting the existence of shadow galaxies, said: "Observationally, a picture is emerging that there is a lot of dark matter in the Universe and that most galaxies possess a great deal of it.

Lensing may reveal hidden matter

"On the theory side, the cold dark matter theory predicts that there are many low-mass galaxies for every massive one, but we don't see many of them around. That could simply be because very few stars - perhaps none at all - have formed in them."

Dr Trentham is to publish a paper on dark galaxies, along with colleagues Ole Moller and Enrico Ramirez-Ruiz, in a forthcoming issue of the Monthly Notices of the Royal Astronomical Society.

Searching for completely dark galaxies represents a difficult challenge, say the scientists. Success will depend on the nature of the dark matter, which is still unknown.

If made up entirely of fundamental particles, dark galaxies may act as gravitational lenses, distorting the appearance of distant bright galaxies lying behind them.

Cool bodies

The same would be true if the galaxies contained any dead stars, such as white dwarfs, or black holes. Several lensing events in a small area of sky would suggest the presence of a dark galaxy.

If the dark matter included some brown dwarfs - cool bodies half way between planets and stars - their infrared radiation may be detectable.

Another give-away would be to observe material being attracted by the gravitational pull of an invisible dark galaxy.

The scientists believe they have already identified one place where a shadow galaxy might exist using this method.

They noticed that a galaxy called UGC 10214 has a stream of material flowing out of it, as if it is being tugged away by another galaxy. But the stream of material is apparently flowing towards nothing.