We present a theory of photovoltaic(PV) spatial solitons(SS's), that includes the PV contribution of a background beam(BB). The theory predicts that a dark or a bright SS can be formed in the same crystal with positive refractive index perturbation (Δn). If the ratio (R) of the effective Glass constants(EGC) of the BB to that of the signal beam(SB) is1. We have formed a one-dimensional dark PV SS in a crystal, as predicted.