What is the largest known structure in the universe?

The Hercules-Corona Borealis Great Wall, a concentration of galaxies stretching perhaps ten billion light years - which is a substantial fraction of the observable universe and is exactly the problem with it.

Cosmology assumes the universe is homogeneous at large scales, and models suggest nothing should be bigger than about 1.2 billion light years, since structure has not had time to grow larger since the Big Bang. Something ten billion across should not exist. The structure was inferred from a clustering of gamma-ray bursts, and several statisticians have argued the sample is too sparse and the apparent grouping is a selection effect rather than a wall.