b'SPACEGravitational LensesIlluminate Dark Matter 16Light from a distant object, such as the galaxy at far right, is gravitationally bent by a massive object or objects closer to Earth. The bending distorts the distant object, potentially creating multiple images of it. Image courtesy NASAI nvisible material called dark matter constitutesresearchers to measure the distribution of matter some 85% of the mass of the universe. In theacross cosmological distances. 1960s and 1970s, Carnegies Vera Rubin, with Kent Ford, confirmed that dark matter exists. TodayThe astronomers looked at special cases of we still do not know what dark matter is. Somegravitational lensingin which a distant galaxys models are good at explaining it at large scales, butlight is distorted by a closer galaxy, producing four not at small scales. Carnegies Andrew Benson, withimages. This effect helps probe the mass function postdoctoral fellow Xiaolong Du and student Turnerand density on subgalactic scales. Using general Johnson, are combining dark matter observationsrelativity, they could predict the relative brightnesses with theoretical modeling to analyze down to aof the four images, which tells them about mass. In scale of 10 7times the mass of our Sunsmall insome cases, the measured brightnesses did not agree astronomical terms.with predictions pointing to extra masssmall dark matter halos. Additionally, they can learn how many The team used a phenomenon called gravitationalhalos there are and where they are. The number and lensing, in which matter like galaxy clusters betweenlocation depend on the properties of the dark matter a distant light source and Earth bends the light.particlefor instance, the lower the particles mass, The amount of bending is predicted by Einsteinsthe fewer the small halos.general theory of relativity. This lensing enables the'