  Motivated by recent observations from Pamela, Fermi and H.E.S.S., we consider
dark matter decays in the framework of supersymmetric SU(5) grand unification
theories. An SU(5) singlet S is assumed to be the main component of dark
matters, which decays into visible particles through dimension six operators
suppressed by the grand unification scale. Under certain conditions, S decays
dominantly into a pair of sleptons with universal coupling for all generations.
Subsequently, electrons and positrons are produced from cascade decays of these
sleptons. These cascade decay chains smooth the electron/positron spectrum,
which permit naturally a good fit to the Fermi LAT data. The observed positron
fraction upturn by PAMELA can be reproduced simultaneously. We have also
calculated diffuse gamma-ray spectra due to the electron/positron excesses and
compared them with the preliminary Fermi LAT data from 0.1 GeV to 10 GeV in the
region 0<l <360, 10<|b|<20. The photon spectrum of energy above 100 GeV, mainly
from final state radiations, may be checked in the near future.
