Context : We calculate constraints from current and future cosmic microwave background ( CMB ) measurements on annihilating dark matter ( DM ) with masses below the electroweak scale : m _ { { DM } } = 5 - 100 GeV .
In particular , we assume the S-wave annihilation mode to be dominant , and focus our attention on the lower end of this mass range , as DM particles with masses m _ { { DM } } \sim 10 GeV have recently been claimed to be consistent with the CoGeNT and DAMA/LIBRA results , while also providing viable DM candidates to explain the measurements of Fermi and WMAP haze .

Aims : We study the model ( in ) dependence of the CMB power spectra on particle physics DM models , large-scale structure formation and cosmological uncertainties .
We attempt to find a simple and practical recipe for estimating current and future CMB bounds on a broad class of DM annihilation models .

Methods : We use a model-independent description for DM annihilation into a wide set of Standard Model particles simulated by PYTHIA Monte Carlo .
Our Markov chain Monte Carlo calculations used for finding model constraints involve realistic CMB likelihoods and assume a standard 6-parameter \Lambda CDM background cosmological model , which is extended by two additional DM annihilation parameters : m _ { { DM } } and \langle \sigma _ { A } \upsilon \rangle / m _ { { DM } } .

Results : We show that in the studied DM mass range the CMB signal of DM annihilations is independent of the details of large-scale structure formation , distribution , and profile of DM halos and other cosmological uncertainties .
All particle physics models of DM annihilation can be described with only one parameter , the fraction of energy carried away by neutrinos in DM annihilation .
As the main result we provide a simple and rather generic fitting formula for calculating CMB constraints on the annihilation cross section of light WIMPs .
We show that thermal relic DM in the CoGeNT , DAMA/LIBRA favored mass range is in a serious conflict with present CMB data for the annihilation channels with few neutrinos , and will definitely be tested by the Planck mission for all possible DM annihilation channels .
Also , our findings strongly disfavor the claim that thermal relic DM annihilations with m _ { { DM } } \sim 10 GeV and \langle \sigma _ { A } \upsilon \rangle \sim 9 \times 10 ^ { -25 } { cm } ^ { 3 } { s } ^ { -1 } could be a cause of Fermi and WMAP haze .

Conclusions :