What is the biggest thing in the Universe?

 The Hercules–Corona Borealis Great Wall is the largest known super-structure in the universe.

It is a huge group of galaxies forming a giant sheet-like pattern. It is about 10 billion light-years long, 7.2 billion light-years wide, and almost 1 billion light-years thick. It is about 10 billion light-years away in the constellations of Hercules and Corona Borealis, hence its name.