Who discovered dark matter in galaxies? Here’s a thread based on our “A history of dark matter” (see arxiv.org/abs/1605.04909 for details and all references) 
In the 1960s radio astronomers measured the rotation curve of the Andromeda galaxy, out to a relatively large radius. See the data obtained by Mort Roberts in 1966
In 1970 Vera Rubin and Kent Ford obtained optical data of M31 out to 110 arcminutes away from the galaxy’s center, and found rotational velocities compatible with radio measurements
In the same year, Ken Freeman compared the radius at which the rotation curve was predicted to peak, to the observed 21 cm rotation curve, and concluded:
D. Rogstad and G. Shostak in 1972 showed that the rotation curves of five galaxies remained flat out to the largest radii observed, confirming “the requirement of low-luminosity material in the outer regions of galaxies”
Morton Roberts was among the first to recognize the implications of the observed flatness of galactic rotation curves. In 1973, together with Arnold Rots, he wrote:
By 1974, flat rotation curves obtained by radio astronomers had done much to establish the existence of large amounts of mass in outer parts of galaxies. This is the incipit of an influential article published in 1974
Portions of the astronomical community, however, were still not convinced. In the late 1970s, this evidence was strengthened and corroborated by new studies. In 1977, Krumm and Salpeter found flat rotation curves for 6 galaxies, but data turned out to be unreliable
In 1978, Albert Bosma published in his PhD thesis the radio observation of the rotation curves of 25 galaxies, proving that most of these objects had flat rotation curves out to the largest observed radius
A few months later, Rubin, Ford and Norbert Thonnard published optical rotation curves for ten high-luminosity spiral galaxies and found that they were flat out to the outermost measured radius
The work of Rubin et al has become the most well-known (one could even say the *only* well-known), despite optical measurements did not extend to radii as large as those probed by radio observations.
The community amnesia towards radio pioneers is however not the fault of Rubin et al, who actually graciously acknowledged the credit that was due to the preceding analyses:
To conclude, note that this "mass discrepancy" problem discovered in galaxies is part of a much bigger problem, which includes a wealth of other cosmological observations, and which led in the 1980s to establish the so-called cold dark matter paradigm
