# CMB anisotropies without dark matter

One of the strongest hints of dark matter are the CMB anisotropies and the peaks. What should a non-dark matter theory do in order to reproduce that evidence? I mean, what is the feature that a pure theory of non-dark matter should satisfy in order to reproduce the CMB results?

• If gravity looks different on galactic length scales, that would do it. – probably_someone Nov 22 '17 at 18:03
• That would explain flat rotation curves, would it explain the CMB anisotropies? – riemannium Nov 22 '17 at 18:45
• This seems rather broad, considering the wide variety of non-DM theories. – Kyle Kanos Nov 22 '17 at 23:45

The main problem is that large scale structure observations tell us that non-relativistic matter makes up about 30% of the energy density of the Universe ($\Omega_m\sim 0.3$). If this was all baryonic then that would make the CMB acoustic peaks much too large relative to the low-$\ell$ Sachs-Wolfe plateau. Additionally, nuclear synthesis would be violated as the primordial abundances tell us that baryons only make up about 5% of the energy density of the Universe ($\Omega_b\sim 0.05$).