  PAMELA's observation that the cosmic ray positron fraction increases rapidly
with energy implies the presence of primary sources of energetic
electron-positron pairs. Of particular interest is the possibility that dark
matter annihilations in the halo of the Milky Way provide this anomalous flux
of antimatter. The recent measurement of the cosmic ray electron spectrum by
the Fermi Gamma Ray Space Telescope, however, can be used to constrain the
nature of any such dark matter particle. In particular, it has been argued that
in order to accommodate the observations of Fermi and provide the PAMELA
positron excess, annihilating dark matter particles must be as massive as ~1
TeV or heavier. In this article, we revisit Fermi's electron spectrum
measurement within the context of annihilating dark matter, focusing on masses
in the range of 100-1000 GeV, and considering effects such as variations in the
astrophysical backgrounds from the presence of local cosmic ray accelerators,
and the finite energy resolution of the Fermi Gamma Ray Space Telescope. When
these factors are taken into account, we find that dark matter particles as
light as ~300 GeV can be capable of generating the positron fraction observed
by PAMELA.
