ABSTRACT I develop a high-velocity galaxy collision model to explain a rare but puzzling phenomenon, namely the apparent existence of ultra-diffuse galaxies with little dark matter. Predictions include simultaneous triggering of overpressured dense clouds to form luminous old globular clusters, a protogroup environment to generate high relative velocities of the initially gas-rich galaxies in the early universe, and spatially separated dark haloes, possibly detectable via gravitational lensing and containing relic low-metallicity stars with enhanced α/Fe at ultra-low surface brightness.