Ars Technica: Galaxy clusters are some of the largest gravitationally bound structures in the universe and are usually found where the large-scale structures of the galaxy intersect. Previous observations of the Abell galaxy cluster revealed several filament-like structures extending from the cluster at various angles, with some physically connected to Abell through clouds of gas. With the European Space Agency's XMM-Newton x-ray observatory, Dominique Eckert of the University of Geneva, Switzerland, and her colleagues used weak gravitational lensing to estimate the masses of those structures. By also measuring the temperature and density of the gases in the structures, the researchers found that the structures had an unexpectedly high mass of hot gas—ranging from 5% to 15%—with most of the rest of the mass coming from unknown dark matter. If a similar concentration of baryonic matter is found in other structures surrounding galaxy clusters, it would be a strong confirmation of the levels of baryonic matter predicted by maps of the cosmic microwave background radiation. However, Eckert notes that a much larger sample of observations is needed, especially of less-dense filamentary structures, before any conclusions can be drawn about the universe as a whole.
