Transcript: The flat rotation curve of the Milky Way has profound implications for the mass distribution of our galaxy. In the solar system the circular orbits of the planet decline with increasing distance from the Sun in accordance with Kepler’s Law and with the idea that the Sun contains essentially all the mass in the solar system. In the Milky Way the situation is entirely different. The velocities are constant or rising with increasing distance, and the implied mass scales proportionally to distance. Out to the Sun’s radius the mass of the Milky Way is two times ten to the eleven solar masses. Out to the edge of the halo measured by halo stars, forty kiloparsecs, the mass is five times ten to the eleven, and from the motions of satellite dwarf companions to the Milky Way to a distance of a hundred kiloparsecs the mass is two times ten to the twelve solar masses, two trillion times the mass of the Sun. Thus most of the mass in the Milky Way galaxy is far out beyond the region of the disk, and most of it corresponds to material that does not emit light at any wavelength raising the issue of dark matter.