Dark matter ring in the galaxy cluster CI 0024+17, Hubble Space Telescope image. The ring is seen surrounding a central concentration of dark matter (coloured blue here). Dark matter does not emit or reflect light, so is hard to detect, but it is thought to make up much of the universe. The distribution of dark matter in the cluster was worked out by observing the effects of the cluster's gravity on the appearance of galaxies far beyond it, a phenomenon known as gravitational lensing. The distant galaxies are seen as distorted blue smears, while the cluster galaxies are bright yellow. Data gathered by the Advanced Camera for Surveys on the HST in November 2004. See R932/062 for the cluster in normal light, and R900/106 for a graphic showing the ring's formation.