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.

At the center of galaxy NGC 1275, some 200 million light years from Earth, there might be a supermassive black hole. In a attempt to map the galaxy and look deep into its heart, an international team of astronomers has linked radio telescopes throughout Europe and America in order to produce a picture of NGC 1275. Jessie Greenstein, the co-discoverer of quasars, and Italian radio astronomer Tiziana Venturi appear.