The CfA2 Great Wall was the structure that made the universe look unexpectedly organized: a coherent sheet of galaxy concentrations stretching roughly 750 million light-years across the CfA2 redshift survey, discovered when the survey's slices revealed galaxies arranged in walls and voids rather than scattered field (Gott et al. 2005; Einasto et al. 2011). It remains one of the largest known structures in the nearby universe.
The problem is arithmetic. Structure in ΛCDM grows hierarchically from inflationary Gaussian fluctuations at calculable rates, and those rates set a ceiling on how large a coherent structure can assemble in the age of the universe, conventionally a few hundred megaparsecs. The Great Wall presses against that ceiling, and its larger siblings, the Sloan Great Wall and the Hercules-Corona Borealis Wall, sail past it. Coherence is the sticking point: gravity can correlate matter only as far as it has had time to act, so a wall whose parts hang together over distances light itself has barely crossed since recombination needs either initial conditions with more large-scale organization than inflation provides, or a formation mechanism the model does not contain.
The standing is part of the giant-structures family of anomalies, each individually arguable, collectively pointed. Statistical defenses, that walls are pattern-recognition artifacts in sparse data, weaken as surveys deepen and the structures persist. DESI and Euclid will map the wall population across far greater volumes, measuring whether coherent structure truly ends at the hierarchical ceiling or keeps going.