  We discuss challenges to a dark matter annihilation interpretation of the
excess positron fraction in the cosmic rays observed by the PAMELA, Fermi-LAT
and AMS-02 collaborations. The spectra of positrons from annihilations into the
leptonic two-body final states like $\tau^+ \tau^-$ or $\mu^+ \mu^-$, fit well
the AMS-02 data for the positron ratio and the electron flux. Furthermore, we
discuss the hadronic annihilation channels $b \bar{b}$ and $W^+ W^-$. However,
this interpretation requires a very large annihilation cross section especially
for the hadronic channels, conflicting with the unitarity upper limit, unless
the positron flux due to annihilations is boosted by a large factor due to
inhomogeneities in the galactic halo. In addition, we present predictions
within this interpretation for the positron fraction at higher energies for the
antiproton flux, and discuss constraints from $\gamma$-ray measurements.
