If you read Fritz Zwicky's original
1933 and 1936 papers, that's what he actually said about Dark Matter.
Einer Expansion von 500 km/sek pro Million Parseks entspricht nach EINSTEIN und DE SITTER eine mittlere Dichte von rho = 10^-28 gr/cm^3. Aus den Beobachtungen an selbstleuchtender Materie schätzt HUBBLE rho ~ 10^-31 gr/cm^3. Es ist natürlich möglich, dass leuchtende plus dunkle (kalte) Materie zusammengenommen eine bedeutend höhere Dichte ergeben, und der Wert rho ~ 10^28 gr/cm^3 erscheint daher nicht unvernünftig.
Helvetica Physica Acta, Vol. 6, p. 122
An expansion rate of 500 kilometers per second per million parsecs is equivalent to an average density of rho = 10^28 grams per cm^3, according to EINSTEIN and DE SITTER. From the observation of self radiating matter HUBBLE estimates rho ~ 10^31 grams per cm^3. Of course, it is possible, that radiating plus dark (cold) matter put together result in a massively larger density, and the value rho ~ 10^28 grams per cm^3 seems not unreasonable.
As you can see, Zwicky himself coins the term Dark Matter as the place holder for non-radiation stuff out there to resolve a discrepancy of 1 to 1000. Better measurements of the Hubble constant, which at the time was estimated to be 500 km/sec per million parsecs, and is now estimated to be 70 km/sec per million parsecs has shrunk the discrepancy to about 1 to 5, but still, Dark Matter is exactly that, postulated stuff to make up for a discrepancy.