Anisotropy of Cosmic Background Photons from Annihilating/Decaying Dark Matter

We provide a detailed formulation for calculating the angular power spectrum of the cosmic background photons arising from the dark matter decay or annihilation in a comprehensive manner. We pay particular attention to the case of dark matter decaying or annihilating into line photons. It is pointed out that taking account of the energy resolution of the detector is essential to correctly evaluate the angular power spectrum. We apply our formulation to the observational data from infrared, optical, X-ray and gamma-ray telescopes.