Indirect detection of composite asymmetric dark matter

Dark matter can form bound states upon the emission of quanta of energy equal to the binding energy. The rate of this process is large for strongly interacting dark matter, and further enhanced by long-distance effects. The resulting monochromatic and diffuse γ rays can be tested in indirect-detection experiments. If dark matter has electroweak charge, indirect signals include multiple observable photon lines for masses in the TeV range. Otherwise, if it couples only via a dark photon portal, diffuse spectra from dwarf galaxies and cosmic microwave background reionization set powerful limits for masses below a tera-electron volt. This mechanism provides a powerful means of probing asymmetric dark matter today.