  A model has been proposed in which neutral scalar particles chi, of mass 1-10
MeV, annihilate through the exchange of a light vector boson U, of mass 10-100
MeV, to produce the 511 keV line observed emanating from the center of the
galaxy. The chi interacts weakly with normal matter and is a viable dark matter
candidate. If the U-boson couples to quarks as well as to electrons, it could
enhance the branching ratio for the rare decay pi0 -> e+e-. A recent
measurement by the KTeV Collaboration lies three standard deviations above a
prediction by Dorokhov and Ivanov, and we relate this excess to the couplings
of the U-boson. The values are consistent with other constraints and
considerations. We make some comments on possible improvements in the data.
