Zwicky's dark matter discovery: how did he determine galactic equilibrium?

[Earnest Guest]

I'm wondering if someone can give a more detailed history of Dark Matter than what I've found on Wikipedia. How did Fritz Zwicky determine that a galaxy was in 'equilibrium' given the uncertainty of a line-of-sight measurement of velocity? How did he determine the mass given the variables of the mass-to-light (M/L) ratio and the poor ability (at the time) to classify galaxies? I understand a little about the Virtal Theorem, but would appreciate a 'For Dummies' type of description of how it applies to globular clusters. Any additional information about the original problem and the techniques involved would be welcome.

 

[Chronos]

Here is a primer http://arxiv.org/abs/astro-ph/?9904251, The Early History of Dark Matter.