The presence of dark gaps, a preferential light deficit along the bar minor axis, is observationally well known. The properties of dark gaps are thought to be associated with the properties of bars, and their spatial locations are often associated with bar resonances. However, a systematic study, testing the robustness and universality of these assumptions, is still largely missing. Here, we investigate the formation and evolution of bar-induced dark gaps using a suite of N-body models of (kinematically cold) thin and (kinematically hot) thick discs with varying thick disc mass fractions and different thin-to-thick disc geometries. We find that dark gaps are a natural consequence of the trapping of disc stars by the bar. The properties of dark gaps (such as strength and extent) are well correlated with the properties of bars. For stronger dark gaps, the fractional mass-loss along the bar minor axis can reach up to similar to 60-80 per cent of the initial mass contained, which is redistributed within the bar. These trends hold true irrespective of the mass fraction in the thick disc and the assumed disc geometry. In all our models harbouring slow bars, none of the resonances (corotation, inner Lindblad resonance, and 4:1 ultraharmonic resonance) associated with the bar correspond to the location of dark gaps, thereby suggesting that the location of dark gaps is not a universal proxy for these bar resonances, in contrast with earlier studies. C1 [Ghosh, Soumavo] Max Planck Inst Astron, Konigstuhl 17, D-69117 Heidelberg, Germany. [Gadotti, Dimitri A.] Univ Durham, Ctr Extragalact Astron, Dept Phys, South Rd, Durham DH1 3LE, England. [Fragkoudi, Francesca] Univ Durham, Inst Computat Cosmol, Dept Phys, South Rd, Durham DH1 3LE, England. [Nagpal, Vighnesh] Univ Calif Berkeley, Dept Astron, Berkeley, CA 94720 USA. [Di Matteo, Paola] PSL Res Univ, GEPI, Observ Paris, CNRS, Pl Jules Janssen, F-92195 Meudon, France. [Cuomo, Virginia] Univ Atacama, Inst Astron & Ciencias Planetarias, Ave Copayapu 485, Copiapo 1530000, Atacama, Chile. C3 Max Planck Society; Durham University; Durham University; University of California System; University of California Berkeley; Centre National de la Recherche Scientifique (CNRS); Universite PSL; Observatoire de Paris; Universidad de Atacama