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The discs of all large galaxies show a "flattening" of their discs where the material at the outer edge of the discs rotate at the same rate as the material near the center of the galaxy. The explanation for this is that the galaxy must be embedded in a halo of invisible "dark" matter which, I assume, is also thought to be rotating. The question is: why does this theory assume that all of the material in the "dark" matter disc rotates at the same rate from its outer edge to its center --- rather than having the material at the outer edge rotating more slowly than near the center as would be expected?
Frank
Frank