  The inclusive cross-section of annihilation in flight
$e^+e^-\rightarrow\gamma\gamma$ of 430 MeV positrons with atomic electrons of a
thin diamond target has been measured with the PADME detector at the Laboratori
Nazionali di Frascati. The two photons produced in the process were detected by
an electromagnetic calorimeter made of BGO crystals. This measurement is the
first one based on the direct detection of the photon pair and one of the most
precise for positron energies below 1 GeV. This measurement represents a
necessary step to search for dark sector particles and mediators weakly coupled
to photons and/or electrons with masses ranging from 1 MeV to 20 MeV with
PADME. The measurement agrees with the Next to Leading Order QED prediction
within the overall 6% uncertainty.
