Astronomers using NASA's Hubble Space Telescope have detected a ring of dark matter around the galaxy cluster Cl 0024+17. This is the first time that dark matter has been detected with a unique structure that is different from both the gas and galaxies in the cluster. The results suggest that the ring was formed 1 to 2 billion years ago as a result of a collision with another galaxy cluster.
Dark matter, as its name suggest, cannot be seen by telescopes directly. Instead, we must resort to indirect means, for example, by measuring the effect it has on normal matter and light. To trace the distribution of dark matter in Cl 0024+17, astronomers observed how the light from distant background galaxies were being distorted by the cluster itself. In an effect known as gravitational lensing, the clusters mass distorts and smears the light from background galaxies into arcs and streaks that can be detected by the Hubble Space Telescope. By mapping the distorted light, astronomers can work out the cluster's mass and how the dark matter is distributed in the cluster. Surprisingly, in the case of Cl 0024+17, the dark matter formed a ring, 2.6 million light-years across, around the cluster. This is the first time that dark matter has been detected so largely separated from the galaxies and hot gas that make up the cluster.
It is believed that the dark matter ring was formed as a result of a head-on collision with another cluster approximately 1 to 2 billion years ago. Spectroscopic observations of the cluster's 3D structure seem to provide further evidence that such an event occurred. Furthermore, computer simulations of such an event using theoretical models also produce results which agree with the measured distribution of the dark matter.