Fritz Zwicky was unquestionably the astronomer who first identified the
discrepancy between the Newtonian dynamical mass and the luminous mass in
clusters of galaxies -- the discrepancy which led to his proposal of dark
matter.
But it is not possible to single out an individual who
discovered, by means of astronomical observations, the need for dark matter
on galaxy scales. The issue has a history going back to Oort (1932),
Babcock (1939), Mayall (1951) and Schwarzschild (1954), but the end result
of more than two decades of research was inconclusive.
The breakthrough
came with the development and use of large radio telescopes, both single
dish and interferometers, in the early 1970s as well as the use of image
intensifiers for optical spectroscopy. Here I discuss the relative roles
of
21 cm and optical emission line observations in tracing the extended rotation
curves of spiral galaxies and the implied mass distribution.
I describe the
contributions of the various scientists involved and discuss the sociology of
recognizing an emerging crisis in science. It is a story of revolution
and reaction with implications for the actual nature of the dynamical
discrepancy in galaxies.
