This Hubble Space Telescope composite image superimposes a blue map of the dark matter distribution in the galaxy cluster Cl 0024+17 on a Hubble image of the cluster. The ring is one of the strongest pieces of evidence to date for the existence of dark matter, an unknown substance that pervades the universe. ((NASA, ESA, M.J. Jee and H. Ford, Johns Hopkins University))

Scientists say they have found the strongest evidence yet of the existence of dark matter, the elusive substance thought to be a key component of the universe.

Images collected by the Hubble space telescope of the collision of two galaxy clusters more than five billion light years away have revealed subtle signs of gravity from an unseen mass at work in the cluster.

When this unseen mass was mapped in blue and superimposed on an image of the combined cluster,a ghostly "ring" that NASA scientists say is a map of dark matter became evident.

Dark matter is impossible to see directly because it doesn't shine or even reflect light and doesn't appear to interact with regular matter.

But its influence in the universe is felt because its mass exerts a gravitational pull on galaxies, helping them to keep their shape and stay together. Based on the movements of stars and galaxies, scientists speculate that dark matter is five to six times more common than the regular matter we know, which is made up of familiar atomic particles such as protons, electrons and neutrons.

Myungkook James Jee, an associate research scientist at Johns Hopkins University, led the team of researchers that included contributions from NASA and the European Space Agency. The findings will be reported in an upcoming issue of the Astrophysical Journal.

Jee said the relatively uniform ring pattern of the dark matter is the result of the impact between the two galaxy clusters that collided to create the current cluster,moving the dark matter inthe same way a pebble dropped in a pond creates a ripple in the water.

Images of galaxies

To come up with the map of unseen matter Jee and his team of researchers looked not at the galaxy clusters themselves but rather the images of galaxies that lay even farther in the distance. The images of these galaxies were stretched, the result of what scientists call gravitational lensing, whereby their light is bent and magnified by the gravity of the galaxy cluster.

Richard Massey, a postdoctoral scholar at California Institute of Technology, who was not part of the research, said the discovery is exciting if it's correct.

"But in order to be convinced, astronomers would need to see it at a different angle or with another camera," he said during a NASA teleconference of the announcement.

Unfortunately for astronomers, such confirmation will have to wait. In January, Hubble's Advanced Camera for Surveys — which took the images — lost its power supply, and a space shuttle crew is not scheduled to repair the camera until September 2008 at the earliest.

Massey called the failure of the camera "heart-breaking," saying it or an instrument like it is necessary to get the detailed images needed to prove the existence of dark matter.

"It's the most common stuff in the universe and we know nothing about it, which is embarrassing," Massey said.