Probably galactic filaments. The largest known structure in the observable Universe is the Sloan Great Wall at a length of around 1.37 billion light years.
We won't know the answer to that question until 1). We know that we've discovered every planet in the universe, and 2). We know the size of every planet that we've discovered.
The largest diamond in the universe is known as lucky. The weight of the precious stone reportedly makes up ten billion trillion trillion carats or five million trillion trillion pounds). The space...