  The vector $U$-bosons, or so called 'dark photons', are one of the possible
candidates for the dark matter mediators. They are supposed to interact with
the standard matter via a 'vector portal' due to the $U(1)-U(1)^\prime$
symmetry group mixing which might make them visible in particle and heavy-ion
experiments. While there is no confirmed observation of dark photons, the
detailed analysis of different experimental data allows to estimate the upper
limit for the kinetic mixing parameter $\epsilon^2$ depending on the mass $M_U$
of $U$-bosons which is also unknown. In this study we present theoretical
constraints on the upper limit of $\epsilon^2(M_U)$ in the mass range $M_U \le
0.6$ GeV from the comparison of the calculated dilepton spectra with the
experimental data from the HADES Collaboration at SIS18 energies where the dark
photons are not observed. Our analysis is based on the microscopic
Parton-Hadron-String Dynamics (PHSD) transport approach which reproduces well
the measured dilepton spectra in $p+p$, $p+A$ and $A+A$ collisions.
Additionally to the different dilepton channels originating from interactions
and decays of ordinary matter particles (mesons and baryons), we incorporate
the decay of hypothetical $U$-bosons to dileptons, $U\to e^+e^-$, where the
$U$-bosons themselves are produced by the Dalitz decay of pions $\pi^0\to
\gamma U$, $\eta$-mesons $\eta \to \gamma U$ and Delta resonances $\Delta \to N
U$. Our analysis can help to estimate the requested accuracy for future
experimental searches of 'light' dark photons by dilepton experiments.
