Dark matter is a mysterious and unknown substance that dominates the universe. Ordinary matter, which is the source of things such as stars, planets, and gas clouds that can be seen, makes up only a few percent of the matter in the universe. Scientists have believed that, unless their understanding of physics is incorrect, dark matter is the main substance in the universe. But until now, they had no evidence of it existing apart from galaxies and hot gas.
An image taken by the Hubble Space Telescope shows a ring of dark matter surrounding a galaxy cluster named ZwCl0024+1652, which lies 5 billion light-years away. The ring measures 2.6 million light-years across. Previous research showed that the galaxy cluster was formed when two separate galaxy clusters collided. Computer simulations revealed what happened after the collision. The dark matter in the two clusters fell to the center before rebounding. But as they moved outward, they were prevented from escaping by gravity, which slowed the dark matter down and piled it up in a ring around the newly formed cluster.
Galaxies would not stay grouped together if dark matter wasn’t present. The visible, everyday matter in galaxy clusters would be insufficient to keep them from breaking apart.
Astronomers cannot “see” dark matter directly.
They infer the existence of this mysterious substance by the effect it has on its surroundings. One of these effects is the gravitational lensing it causes. Gravitational lensing is when massive foreground objects bend the light from more distant galaxies. Images reveal this bending as streaks of light stretched in arcs around the massive object. Mapping the distorted light allows the foreground object’s mass and distribution to be determined. The discovery of this ring-shaped object around galaxy cluster ZwCl0024+1652 is the best evidence yet that dark matter exists.
M. James Jee of Johns Hopkins University in Baltimore was one of the team members that discovered the dark-matter ring. “I was annoyed when I saw the ring because I thought it was an artifact, which would have implied a flaw in our data reduction,” he says. I couldn’t believe my result. But the more I tried to remove the ring, the more it showed up. It took more than a year for me to convince myself that the ring was real. I’ve looked at a number of clusters and I haven’t seen anything like this. “
As Jee describes the importance of the discovery: “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. 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.”