In 1933, Zwicky reported his measurement of the velocity dispersion of galaxies in the Coma cluster of galaxies. Using virial theorem techniques that are used today, he showed that the mass of the cluster is much larger than the likely sum of the masses of the individual galaxies. A similar result was found by Smith (1936) for the Virgo cluster. Zwicky's value for the velocity dispersion of Coma is close to present estimates. This profound discovery got little response, other than Smith's follow-up study for Virgo.
It took another 35 years for the saga of dark matter (DM) in galaxies to take off. And then it took another 20 years for us to learn that clusters of galaxies have the universal baryon content (about 16% of their mass, including the hot gas which dominates the baryon component of the cluster), and that the rest of their mass is in the form of DM which is believed to reside partly in the galaxies and partly in the cluster itself (White et al 1993).
Why did Zwicky's important and apparently straightforward discovery get so little response ? Did the community already regard Zwicky with suspicion (or did that come later) ? Or was it due to suspicion of results for which there is no existing theoretical framework ? Some would agree with this irritating quote from Eddington (1947): “It is … a good rule not to put overmuch confidence in observational results until they are confirmed by theory”. (I am grateful to Matt Stanley for providing the source of this quote.) Why didn't astronomers follow up Zwicky's important discovery with spectroscopy of nearby groups of galaxies like the M81 and Leo groups, which would be much easier to study than the more distant galaxies in the Coma cluster?
Zwicky does now get full credit for his discovery. This is a partial counter-example to Stigler's Law of Eponymy, that no scientific discovery is named after its original discoverer. Big new ideas only tend to catch on when the scientific community is ready for them.