  The Belle II collaboration has recently observed the rare decay $B^+\to K^+
\nu \bar{\nu}$, finding an excess with respect to the Standard Model
prediction. We explore the possibility that the data entails long-distance
interactions induced by a massless dark photon,
$\gamma_{\scriptscriptstyle{D}}$. This couples at the tree-level to an
invisible, dark sector and to the Standard Model via higher-dimensional
operators, such as the chromomagnetic-dipole coupling that we use to explain
the excess. As the process $B^+\to K^+ \gamma_{\scriptscriptstyle{D}}$ is
forbidden by angular momentum conservation, the transition mediated by the
off-shell dark photon yields a three-body final state comprising a pair of dark
fermions that show as a missing energy continuum in the detector, faking the
neutrino signature. We show that the Belle II data is explained for
perturbative values of the parameters of the model. This scenario predicts new
contributions to the neutral $B$ meson decays $B^0\to K^*
\gamma_{\scriptscriptstyle{D}}$, in which the emission of a on-shell dark
photon is allowed, yielding a monochromatic missing energy signature.
Analogously, an excess due to the emission of a dark photon is predicted for
the $B^0_s\to \phi + E_{\rm miss}$ decay that could be scrutinized next at the
LHCb experiments.
