Fig. and Fig. present the polariton wavepacket and the dark exciton density in position space for broadband excitations. Over time, the UP (blue) and LP wavepacket (red) propagates outward from the center, primarily due to their photonic character which exhibit ballistic transport (with v g largely adopted from the derivative of the band). Due to the exciton-phonon coupling, the UP and LP wavepackets transfer population to the dark state, resulting in an increase in the dark state (black) probability densities. The resultant dark-state wavepacket is largely immobile, as it consists of excitons that move diffusively in most transport experiments. In current theoretical studies, these dark states are completely immobile because of their dispersionless band assumed in the model (Fig. ). However, we note that in some materials, even exciton exhibits wavelike transport at room temperature, and the role of the dark states need to be carefully examined for those materials when coupling them inside the cavity.