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The recently discovered 3.5 keV X-ray line from extragalactic sources may beevidence of dark matter scatterings or decays. We show that dark atoms can bethe source of the emission, through their hyperfine transitions, which would bethe analog of 21 cm radiation from a dark sector. We identify two families ofdark atom models that match the X-ray observations and are consistent withother constraints. In the first, the hyperfine excited state is long-livedcompared to the age of the universe, and the dark atom mass is relativelyunconstrained; dark atoms could be strongly self-interacting in this case. Inthe second, the excited state is short-lived and viable models areparameterized by the value of the dark proton-to-electron mass ratio $R$: for$R = 10^2-10^4$, the dark atom mass is predicted be in the range $350-1300$GeV, with fine structure constant $\alpha'\cong 0.1-0.6$. In either class ofmodels, the dark photon must be massive with $m_{\gamma'} \gtrsim$ 1 MeV anddecay into $e^+ e^-$. Evidence for the model could come from direct detectionof the dark atoms. In a natural extension of this framework, the dark photoncould decay predominantly into invisible particles, for example $\sim 0.5$ eVsterile neutrinos, explaining the extra radiation degree of freedom recentlysuggested by data from BICEP2, while remaining compatible with BBN.

3.5 keV X-rays as the "21 cm line" of dark atoms, and a link to light sterile neutrinos