Astronomers have found one of the best pieces of evidence for the existence of dark matter, a mysterious quantity that pervades our Universe.
They have identified what appears to be a ghostly ring in the sky which is made up of this enigmatic substance.
Using the Hubble Space Telescope, the scientists have established that the ring formed long ago after a colossal smash-up between two galaxy clusters.
Details of the research are to appear in the Astrophysical Journal.
As the name suggests, dark matter does not reflect or emit detectable light, yet it accounts for most of the mass in the Universe.
Astronomers have long suspected the existence of this invisible "stuff" as the source of additional gravity that holds together galaxy clusters.
The clusters would fly apart if they were reliant only on the gravity from their visible stars.
No one knows what dark matter is made of, but it is thought to be a type of elementary particle found throughout the cosmos.
Researchers from Johns Hopkins University and the Space Telescope Science Institute - both in Baltimore, US - spotted the ring unexpectedly while they were mapping the distribution of dark matter within the galaxy cluster Cl 0024+17.
This cluster lies 5 billion light-years from Earth; its ring of dark matter measures 2.6 million light-years across.
Because astronomers cannot see dark matter, they must infer its existence by studying how its gravity bends the light of more distant, background galaxies.
This powerful trick, called gravitational lensing, allows astronomers to map the distorted light to deduce the cluster's mass and how dark matter is distributed in the cluster.
At first, team members thought the ring was an illusion - or artefact - in the data. But repeated attempts to make the ring disappear met with failure. Finally, the astronomers became convinced that it must be a real feature.
Ripples in a pond
In August 2006, US astronomers identified the gravitational signature of dark matter in another merging galaxy cluster. But the ring structure in Cl 0024+17 is exceptional.
"Although the invisible matter has been found before in other galaxy clusters, it has never been detected to be so largely separated from the hot gas and the galaxies that make up galaxy clusters," said co-author Myungkook James Jee of Johns Hopkins University.
"By seeing a dark-matter structure that is not traced by galaxies and hot gas, we can study how it behaves differently from normal matter."
Computer simulations of galaxy cluster collisions show that when two clusters smash together, the dark matter falls to the centre of the merged cluster and sloshes back out.
As the dark matter seeps outward, it begins to slow down under the pull of gravity and gathers together like a traffic pile-up.
Luckily, astronomers had a head on view of this collision because it occurred along the Earth's line of sight.
"It's like looking at the pebbles on the bottom of a pond with ripples on the surface. The pebbles' shapes appear to change as the ripples pass over them," Dr Jee explained.
"So, too, the background galaxies behind the ring show coherent changes in their shapes due to the presence of the dense ring."
Team member Holland Ford, also of Johns Hopkins, said: "By studying this collision, we are seeing how dark matter responds to gravity.
He added: "Nature is doing an experiment for us that we can't do in a lab, and it agrees with our theoretical models."