Astronomers have discovered an object that appears to be an invisible galaxy made almost entirely of dark matter.
The invisible galaxy could only be "seen" using radio waves
The team, led by Cardiff University, UK, claims it is the first such object to be detected.
A dark galaxy is an area in the Universe containing a large amount of mass that rotates like a galaxy, but contains no stars.
It was found 50 million light-years away using radio telescopes in England and Puerto Rico.
Very little is known about "dark matter", even though there is much more of it in the cosmos than "normal", or baryonic, matter, which constitutes the visible material from which stars and planets are built.
The five-year research has involved 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, the research team found a mass of hydrogen atoms a hundred million times the mass of the Sun.
This hydrogen is thought to take the form of a flat disc of rotating material - which is what is seen in ordinary spiral galaxies.
The mysterious new galaxy has been called VIRGOHI21.
Similar objects that have previously been discovered have since turned out to contain stars or be remnants of two galaxies colliding.
However, the scientists from the UK, France, Italy and Australia found no visible trace of any stars, and no galaxies nearby that would suggest a collision.
More to come
Dr Robert Minchin, of Cardiff University, said: "From its speed, 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."
Astronomers say the discovery marks an important breakthrough because, according to cosmological models, dark matter is five times more abundant than the baryonic matter.
We only know of dark matter's existence because of its influence on ordinary matter.
Scientists can infer its presence by looking at the rotation of galaxies and measuring how fast their visible components are moving.
The amount of matter in a galaxy dictates the gravitational force needed to hold it together.
Astronomers have seen galaxies where the material is moving so fast that they should fly apart - as they don't, there must be a stronger gravitational force acting than can be accounted for using visible matter.
Another of the Cardiff team, Dr Jon Davies, added: "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."