for scene 2 of the video " Dark Matter: 'Discovered'" by Rolf Witzsche
here for a larger image
Postulated to account for the stark discrepancies
The existence of dark matter was first postulated in the early 1930s to account for the stark discrepancies between observed 'orbital' velocities of stars in the Milky Way and the laws of orbital mechanics discovered by Johannes Kepler in the early 1600s.
In our solar system the orbital velocity of planets diminishes with the square of the distance. The velocity of the planets at the lower left reflects the gravity forced dynamics of orbital motion that Johannes Kepler had discovered. With the force of gravity diminishing with the square of the distance, the outer planets move more slowly, because if they didn't, the centrifugal force of their movement would drive them out of the solar system.
Ironically, the opposite relationship between speed and distance has been measured in the motion of stars. It has been reasoned from this observation that the mass distribution in a galaxy must be vastly different than it is in a solar system, because stellar orbits are simply not possible under the laws that Kepler had dealt with..
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Published by Cygni Communications Ltd.
North Vancouver, BC, Canada - (C) in public domain - producer Rolf A. F.