  The observed deviations from the Standard Model in several b -> s mu mu
processes can be explained in terms of a new vector boson produced on-shell in
B meson decays. A mass of 2.5-3 GeV and a total width of 10-20% allow to hide
the associated dimuon bump in the poorly known charmonium region, and the large
invisible decay width can be interpreted in terms of Dark Matter. This proposal
predicts a contribution to the muon anomalous magnetic moment, that could
explain the long-standing tension with the Standard Model. It also predicts
sizeable invisible B decays and a peculiar q^2-dependence of the lepton flavor
universality ratios R_K and R_K* , that could be tested at the LHCb and
Belle-II. This proceeding is based on arXiv:1704.06188, and slightly extends it
with comments about Dark Matter.
