Astronomers believe that most of the matter in the universe is invisible to us — so called “dark matter.” The nature of this dark matter is not known, but most astronomers think that it is in the form of an as-yet-unknown type of elementary particle that contributes to gravity through its mass but otherwise interacts weakly with normal matter. These dark matter particles are often called WIMPs, an acronym for “weakly interacting massive particles.” Using the Chandra X-ray Observatory, a research team observed five galaxy clusters to determine how much dark matter they contain. Galaxy clusters are vast concentrations of galaxies, hot gas and dark matter spanning millions of light years, held together by gravity. Because of their size, clusters of galaxies are thought to provide a fair sample of the proportion of dark matter in the universe as a whole. “We found that the stars in the galaxies and hot gas together contribute only about 13 percent of the mass. The rest must be in the form of dark matter,” said researcher Steven W. Allen of the Institute of Astronomy in Cambridge, England.