The Planck collaboration just released updated results that include an input from their CMB polarization measurements. The most interesting are the new constraints on the annihilation cross section of dark matter:
Dark matter annihilation in the early universe injects energy into the primordial plasma and increases the ionization fraction. Planck is looking for imprints of that in the CMB temperature and polarization spectrum. The relevant parameters are the dark matter mass and <σv>*feff, where <σv> is the thermally averaged annihilation cross section during the recombination epoch, and feff ~0.2 accounts for the absorption efficiency. The new limits are a factor of 5 better than the latest ones from the WMAP satellite, and a factor of 2.5 better than the previous combined constraints.
What does it mean for us? In vanilla models of thermal WIMP dark matter <σv> = 3*10^-26 cm^3/sec, in which case dark matter particles with masses below ~10 GeV are excluded by Planck. Actually, in this mass range the Planck limits are far less stringent the ones obtained by the Fermi collaboration from gamma-ray observations of dwarf galaxies. However, the two are complementary to some extent. For example, Planck probes the annihilation cross section in the early universe, which can be different than today. Furthermore, the CMB constraints obviously do not depend on the distribution of dark matter in galaxies, which is a serious source of uncertainty for cosmic rays experiments. Finally, the CMB limits extend to higher dark matter masses where gamma-ray satellites lose sensitivity. The last point implies that Planck can weigh in on the PAMELA/AMS cosmic-ray positron excess. In models where the dark matter annihilation cross section during the recombination epoch is the same as today, the mass and cross section range that can explain the excess is excluded by Planck. Thus, the new results make it even more difficult to interpret the positron anomaly as a signal of dark matter.