  The Large Area Telescope onboard the Fermi Gamma-ray Space Telescope has
collected the largest ever sample of high-energy cosmic-ray electron and
positron events. Possible features in their energy spectrum could be a
signature of the presence of nearby astrophysical sources, or of more exotic
sources, such as annihilation or decay of dark matter (DM) particles in the
Galaxy. In this paper for the first time we search for a delta-like line
feature in the cosmic-ray electron and positron spectrum. We also search for a
possible feature originating from DM particles annihilating into
electron-positron pairs. Both searches yield negative results, but we are able
to set constraints on the line intensity and on the velocity-averaged DM
annihilation cross section. Our limits extend up to DM masses of 1.7 $TeV/c^2$,
and exclude the thermal value of the annihilation cross-section for DM lighter
than 150 $GeV/c^2$.
