5.1. No Dark Matter?
Zwicky's original estimate of the large mass of Coma was regarded with considerable scepticism at the beginning (see Section 3.1). Possible solutions to the problem of dark matter were that Coma (and clusters in general) are unbound and expanding, or that the v-estimate was boosted by the presence of interlopers. In the words of Holmberg:
The temporary members with their hyperbolic velocities seem to offer a more plausible solution of the difficulty.
This solution to the problem was never totally discarded, since unidentified subclustering is known to significantly affect cluster v-estimates. In the case of Coma, however, Schwarschild pointed out that interlopers cannot led to more than a factor two overestimate of the cluster v. On the other hand, the instability argument could not explain why we see so many clusters, nor the apparent lack of significant evolution in the structure of clusters at different redshifts (see Section 3.1).
The existence of dark matter was again questioned by Tifft . He discovered a correlation between galaxy redshifts and magnitudes that led him to question the usual physical interpretation of a galaxy redshift. Tifft estimated that the intrinsic Doppler velocity dispersion could be less than 220 km/s. To my knowledge, this so-called "band-effect" has never really been ruled out (it was even recently confirmed by Nanni et al. ), unless Simkin  was right and the effect is an artefact due to night-sky distorsion in the observed spectra.
Another viable solution to the "missing mass" problem, that does not require any dark matter, came from Milgrom  who proposed a modification of the theory of Newton dynamics. Nevertheless, MOND, this new theory, cannot at the same time explain Coma dynamics and the spiral rotation curves (The & White ).