The Great Attractor announced itself through motion before anyone saw it: galaxies across hundreds of millions of light-years, our own Local Group included, stream toward a region 150 to 250 Mly away in the Centaurus-Norma direction, a flow that implies an enormous concentration of mass near the Zone of Avoidance where the Milky Way's disk hides the sky (Kashlinsky et al. 2010; Feldman et al. 2010). Later work folded the Attractor into the larger Laniakea basin and extended the flow toward the Shapley Concentration, but the central problem survived the remapping.
The problem is an accounting failure. When the galaxies and clusters in the Attractor region are counted and weighed, visible matter plus standard dark-matter distributions fall short of the mass needed to drive the observed flow amplitudes. ΛCDM must then choose between an exotic concentration of dark matter that the galaxy distribution does not trace, unrecognized systematics in the velocity or mass surveys, or the possibility that the large-scale velocity field is responding to something other than local gravitational attraction. Each option costs the model something: the first breaks the usual galaxy-traces-mass assumption, the second has to apply across independent datasets, and the third is not available within the framework at all.
The standing is mature but unresolved: the flow is real, repeatedly confirmed, and pointed; the mass budget that should explain it has never closed. Surveys pushing into the Zone of Avoidance and full-basin reconstructions from CosmicFlows data keep sharpening the deficit rather than dissolving it.