The answer might depend on how one defines "structure." A grouping of galaxies known as the Sloan Great Wall was discovered in the Sloan Digital Sky Survey and is a leading candidate.
Galaxies within one billion light years, a redshift of about 0.1, are depicted.
The labeled Sloan Great Wall spans over one billion light years, longer than any structure ever measured.
Critics worry that the Sloan Great Wall should not itself be characterized as a coherent structure because it is not currently gravitationally bound together and parts of it might never become gravitationally bound.
