b'The Presidents CommentaryMeasuring Dark MatterResearchers at the Observatories are continuing Carnegies decades of interrogation into the enigma of dark matterthe invisible form of matter that makes up most of the universes mass and influences its underlying structure. Staff scientist Andrew Benson, postdoctoral fellow Xiaolong Du, and student Turner Johnson are using a phenomenon known as gravitational lensing to detect and measure dark matter across cosmological distances. Gravitational lensing occurs when the 8 gravity of objects such as massive galaxy clusters, which contain huge amounts of dark matter, bends and distorts the light of more-distant galaxies located behind the cluster. The Carnegie group looked at special cases of gravitational lensing, in which the distortion of a distant galaxys light by a closer galaxy produces four images. By measuring the brightness of the images, the astronomers were able to develop models to identify likely concentrations of dark matter, down to a scale of 10 7times the mass of our Sun (amazingly precise, by cosmic standards).Invisible material called dark matter constitutes some 85% of the mass of the universe. In the 1960s and 1970s, Carnegies Vera Rubin, with Kent Ford, confirmed that dark matter exists. But dark matter continues to remain a mystery and is one of the most important astronomical inquiries today. Image courtesy NASA/GSFC'