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Astronomers believe the universe contains at least ten times as much mass as can be seen and accounted for. How are they able to detect this missing mass and to discover more about it are the subjects of this episode. Astronomers Vera Rubin and Tony Tyson describe their search for dark matter, material invisible to us but whose gravity affects the motion of the stars and can alter the light reaching us from distant galaxies . In addition, John Dobson, a self-proclaimed "sidewalk astronomer," shares his knowledge with people he encounters.