  Antimatter cosmic-rays are used to probe new phenomena in physics, including
dark matter annihilation. We use the cosmic-ray positron fraction spectrum by
the Alpha Magnetic Spectrometer, to search for such an annihilation signal in
the Galaxy. We focus on dark matter with mass between 5 and 120 GeV, producing
high-energy electrons and positrons. In these cosmic-ray energies the interplay
of multiple astrophysical sources and phenomena, makes this search highly
sensitive to the underlying astrophysical background assumptions. We use a vast
public library of astrophysical models for the cosmic-ray positron fraction
background, to derive robust upper limits on the dark matter's annihilation
cross section for a number of annihilation channels. This library accounts for
different types of cosmic-ray sources and uncertainties on their distribution
in space and time. Also, it accounts for uncertainties on those sources'
output, their injected into the interstellar medium cosmic-ray spectra and for
uncertainties on cosmic-ray propagation. For any given dark matter particle
mass and annihilation channel, upper limits on the annihilation cross section
are given by bands that stretch a full order of magnitude in its value. Our
work provides weaker limits compared to earlier results, that are however
robust to all the relevant astrophysical uncertainties. Between 5 and 15 GeV,
we find indications for a possible excess flux of cosmic-ray electrons and
positrons. That excess is found for most, but not all of our astrophysical
background parameter space, and its significance can vary appreciably. Further
scrutiny is necessary to improve the understanding of these lower energy cosmic
rays. Finally, we note that even if an excess signal is found in these
energies, the current background uncertainties do not allow us to accurately
deduce its underlying particle properties.
