> The observations in the 1930s you allude to were of the motions of galaxies in galaxy clusters. The rotation-curve measurements for individual galaxies starte

> The observations in the 1930s you allude to were of the motions of galaxies in galaxy clusters. The rotation-curve measurements for individual galaxies started in the 1970s and really took off in the 1980s. Nice catch. > "Tidal effects" are a non-starter, and do nothing to explain galaxy rotation curves. Nor do they explain the excess motions of individual galaxies in galaxy groups and clusters, or the excess pressure of hot, X-ray emitting gas in groups and clusters -- all of which requires either some kind of dark matter, or some kind of alternate theory of gravity, or both. What a well formed and well supported argument, and not just a matter of fact statement that you've left entirely unsupported. I'm totally convinced!