Hacker News Reader

The reddening is because of the additional mass of the dark-matter-containing structures, and also because the distribution of the dark matter into halos allows for slower orbits of stars and gas clouds, thus suppressing their non-gravitational non-cosmological redshifts. (In an edge-on spiral galaxy without dark matter we would see a greater difference in the blueshift of one extreme tip of the thin disk and the redshift of the opposite extreme tip of the thin disk, compared to the same spiral galaxy with dark matter; the light-and-radio from the central bulge would be less redshifted in the absence of dark matter).

This is the exact opposite of the truth. An edge-on spiral galaxy without dark matter would have less extreme observed rotation velocities (comparing the blueshifted and redshifted sides of the disk), because only the visible mass (stars and gas) is relevant. But in practice we see higher rotation velocities (greater difference between blueshifted and redshifted sides) than can be explained by just the visible mass, and so we have to postulate extra mass in the form of "dark matter" in the galaxy.

(Dark matter halos are not just dark matter outside the visible galaxy, they extend all the way to the centers of galaxies, and in fact become denser the closer to the galaxy center you get. Of course, the visible matter also becomes denser the closer to the center you get, and usually faster than the dark matter does, so the central regions of massive galaxies are dominated by the visible matter. But there's still dark matter there, and in some dwarf galaxies, the dark matter even in the central regions.)

View on HN