On direct and indirect searches for dark matter

In addition to gamma rays, dark matter annihilation products can include energetic electrons which inverse Compton scatter with the cosmic microwave background to produce a diffuse extragalactic background of gamma rays and X-rays. In this dissertation we present an estimate of the extragalactic background flux of gamma rays including inverse Compton photons for a number of annihilation channels and review its dependence on such factors as the halo density profile, the mass-concentration ratio and others and find that the uncertainty of the estimate can reach about two orders of magnitude. For models in which the dark matter particles annihilate primarily to electrons or muons, the measurements of Fermi Gamma Ray Telescope and COMPTEL can provide significant constraints on the annihilation cross section.;We also study the effect of dark matter annihilation during the epoch of reionization. Although empirical evidence indicates that the universe's gas had become ionized by redshift z ≈ 6, the astrophysical models of sources of reionization remain poorly constrained. We explore the possibility that dark matter annihilations may have played a significant role in this process. Energetic electrons produced in these annihilations can scatter with the cosmic microwave background to generate relatively low energy gamma rays, which ionize and heat gas far more efficiently than higher energy prompt photons.;On the side of indirect detection, recent observations by the CoGeNT collaboration (as well as long standing observations by DAMA/LIBRA) suggest the presence of a ∼ 5--10 GeV dark matter particle with a somewhat large elastic scattering cross section with nucleons (sigma ∼ 10 -40 cm2). Simple extensions of the MSSM, however, such as the extension of the MSSM by a chiral singlet superfield allow for the possibility that the dark matter is made up of a light singlino that interacts largely through the exchange of a fairly light (∼30--70 GeV) singlet-like scalar higgs. Such a scenario is consistent with all current collider constraints and can generate the signals reported by CoGeNT and DAMA/LIBRA and can naturally provide the thermal relic abundance consistent with the measured density of dark matter.