Astronomers using NASA's Hubble Space Telescope have discovered a ghostly ring of dark matter that formed long ago during a titanic collision between two galaxy clusters.

Dark matter makes up most of the universe's material. Ordinary matter, which makes up stars and planets, comprises only a small percent of the universe's matter. The ring's discovery is among the strongest evidence yet that dark matter exists, NASA announced Tuesday in a statement.

"This is the first time we have detected dark matter as having a unique structure that is different from both the gas and the galaxies in the cluster," said astronomer James Jee of Johns Hopkins University. Jee is a member of the team that spotted the dark matter ring.

The ring, which measures 2.6 million light-years across, was found in the cluster CL0024+17, located 5 billion light-years from Earth.

Curious about why the ring was in the cluster and how it had formed, Jee found previous research that suggested the cluster had collided with another cluster 1 to 2 billion years ago.

Astronomers have a head-on view of the collision because it occurred along Earth's line of sight. From this perspective, the dark-matter structure looks like a ring.

The team created simulations showing what happens when galaxy clusters collide. As the two clusters smash together, the dark matter, as calculated in the simulations, falls to the center of the combined cluster and sloshes back out. As the dark matter moves outward, it begins to slow down under the pull of gravity and pile up.

Previously, observations of the Bullet Cluster with Hubble and the Chandra X-ray Observatory presented a sideways view of a similar encounter between two galaxy clusters. In that collision, the dark matter was pulled apart from the hot cluster gas, but the dark matter still followed the distribution of cluster galaxies.

CL0024+17 is the first cluster to show a dark matter distribution that differs from the distribution of both the galaxies and the hot gas.

The team's paper has been accepted for publication in the June 1 issue of Astrophysical Journal.