Dynamic crack branching instabilities in a brittle material are studied numerically by using a non-local damage model. PMMA is taken as our model brittle material. The simulated crack patterns, crack velocities and dissipated energies, compare favorably to experimental data gathered from the literature, as long as the critical strain for damage initiation as well as the parameters for a rate-dependent damage law are carefully selected. Nonetheless, the transition from a straight crack propagation to the emergence of crack branches is very sensitive to the damage initiation threshold. The transition regime is thus a particularly interesting challenge for numerical approaches. We advocate using the present numerical study as a benchmark to test the robustness of alternative non-local numerical approaches.