  We derive the strongest constraint on the fraction of dark matter that can be
composed of low mass primordial black holes by using the observation of the
Galactic Center 511 keV gamma-ray line. Primordial black holes of masses
$\lesssim$ 10$^{15}$ kg will evaporate to produce $e^\pm$ pairs. The positrons
will lose energy in the Galactic Center, become non-relativistic, and then
annihilate with the ambient electrons. We derive robust and conservative bounds
by assuming that the rate of positron injection via primordial black hole
evaporation is less than what is required to explain the SPI/ INTEGRAL
observation of the Galactic Center 511 keV gamma-ray line. Depending on the
primordial black hole mass function and other astrophysical uncertainties,
these constraints are the most stringent in the literature and show that
primordial black holes contribute to less than 1\% of the dark matter density.
Our technique also probes part of the mass range which was completely
unconstrained by previous studies.
