  A massive black hole is present at the centre of our galaxy and inevitably
accretes dark matter particles, creating a region of very high particle
density. The annihilation rate is enhanced with a large number of e+e- pairs
produced either directly or by successive decays of mesons. We evaluate the
synchrotron emission (and self-absorption) associated with the propagation of
these particles through the galactic magnetic field and give constraints on the
values of mass and cross section of the dark matter particles.
