Our Local Group of galaxies is moving with respect to the cosmic microwave background (CMB) with a velocity 1 of V CMB = 631 ± 20 km s-1 and participates in a bulk flow that extends out to distances of ~20,000 km s-1 or more 2-4 . There has been an implicit assumption that overabundances of galaxies induce the Local Group motion 5-7 . Yet underdense regions push as much as overdensities attract 8 , but they are deficient in light and consequently difficult to chart. It was suggested a decade ago that an underdensity in the northern hemisphere roughly 15,000 km s-1 away contributes significantly to the observed flow 9 . We show here that repulsion from an underdensity is important and that the dominant influences causing the observed flow are a single attractor — associated with the Shapley concentration — and a single previously unidentified repeller, which contribute roughly equally to the CMB dipole. The bulk flow is closely anti-aligned with the repeller out to 16,000 ± 4,500 km s-1. This 'dipole repeller' is predicted to be associated with a void in the distribution of galaxies.