Dark photons are predicted by various new physics models, and are being intensively studied in a variety of experiments. In the first part of this paper, we obtain partial wave unitarity constraints on the dark photon parameter space from the allowed VV→VV scattering processes in the limit of large center-of-mass energy, where V=W,Z. In the second part of the paper, searches are performed using the expected differential rates with a realistic detector simulation including a comprehensive set of background processes on dilepton and dilepton plus a photon events at the High Luminosity LHC. In these searches, sensitive differential distributions are used in an optimized way to determine the sensitivity to dark photon parameter space. It is shown that remarkable sensitivity to the dark photon model is achieved and kinetic mixing strength can be probed down to $(1.4−10)\times 10^{−4}$ for dark photon mass between 15 GeV to 2 TeV. We also investigate the sensitivity of a future muon collider suggested by the Muon Accelerator Program (MAP) to the dark photon model at different center-of-mass energies. It is shown that a future muon collider is able to reach a sensitivity to kinetic mixing at the order of $10^{−4}$.