Although empirical evidence indicates that the Universe’s gas had become ionized by redshift $z\approx 6$, the mechanism by which this transition occurred remains unclear. In this article, we explore the possibility that dark matter annihilations may have played the dominant 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. In contrast to previous studies, we find that viable dark matter candidates with electroweak scale masses can naturally provide the dominant contribution to the reionization of the Universe. Intriguingly, we find that dark matter candidates capable of producing the recent cosmic ray positron excesses observed by PAMELA (or the electrons spectrum measured by the Fermi Gamma Ray Space Telescope) are also predicted to lead to the full reionization of the Universe by $z\sim 6$.

• Received 14 April 2009

DOI:https://doi.org/10.1103/PhysRevD.80.035007