  A search for a dark photon, a new light neutral particle, which decays
promptly into collimated pairs of electrons or muons is presented. The search
targets dark photons resulting from the exotic decay of the Standard Model
Higgs boson, assuming its production via the dominant gluon-gluon fusion mode.
The analysis is based on 140 fb$^{-1}$ of data collected with the ATLAS
detector at the Large Hadron Collider from proton-proton collisions at a
center-of-mass energy of 13 TeV. Events with collimated pairs of electrons or
muons are analysed and background contributions are estimated using data-driven
techniques. No significant excess in the data above the Standard Model
background is observed. Upper limits are set at 95% confidence level on the
branching ratio of the Higgs boson decay into dark photons between 0.001% and
5%, depending on the assumed dark photon mass and signal model.
