  Electron-positron pairs have been observed in the 10.95-MeV $0^-\to0^+$ decay
in $^{16}$O. The branching ratio of the e$^+$e$^-$ pairs compared to the
3.84-MeV $0^-\to2^+$ $\gamma$ decay of the level is deduced to be
$20(5)\times10^{-5}$. This magnetic monopole (M0) transition cannot proceed by
$\gamma$-ray decay and is, to first order, forbidden for internal pair
creation. However, the transition may also proceed by the emission of a light
neutral $0^{-}$ or $1^{+}$ boson. Indeed, we do observe a sharp peak in the
$e^{+}e^{-}$ angular correlation with all the characteristics belonging to the
intermediate emission of such a boson with an invariant mass of 8.5(5)
MeV/c$^2$. It may play a role in the current quest for light dark matter in the
universe.
