The largest known structure in the universe is the Hercules-Corona Borealis Great Wall.
Why it is true
This massive galactic filament is about ten billion light-years long and contains thousands of galaxy superclusters bound by gravity and dark matter.
Source: Cosmology
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.