Stellar Motions
Dutch astronomer Jan Oort first discovered the presence of dark
matter in the 1930's when studying stellar motions in the local galactic
neighborhood.
By observing the Doppler shifts
of stars moving near the galactic plane, Oort was able to calculate
how fast the stars were moving. Since he observed that the galaxy was
not flying apart he reasoned that there must be enough matter around
that the gravitational pull kept the stars from escaping, much as the
sun's gravitational pull keeps the planets in the solar system in orbit.
He was able to determine that there must be three times as much mass
as is readily observed in the form of visible light.
Hence, Oort's calculations yielded an M/L ratio of 3 for the region of the
immediate galactic neighborhood.
The M/L ratio increases by several orders of magnitude as larger
astro-physical phenomena come under similar scrutiny.