Gravity forces galaxies that are relatively close together to form clusters, which in turn form superclusters between vast stretches of cosmic void. But now there's an even bigger level of organization...and we have no idea how to explain it.
In the immediate aftermath of the Big Bang, the universe was basically flat, with only small variations in density as you went from place to place. Gravity went against that trend, as it brings dust and gas together to form stars and planets, and those solar systems then pull together to form galaxies. Obviously, the density of a galaxy is far greater than that of nearly empty space, and that difference becomes more pronounced as you compare clusters and superclusters with the rest of the universe.
Still, for all these increasingly vast levels of organization, the universe should still be fundamentally smooth, meaning that no cosmic structures are greater than a few hundred million light-years in length. That's why the discovery of hyperclusters that are at least three billion light-years across represents such a huge cosmic mystery. These structures aren't much, representing only about a 1% variation in density from the rest of the universe, but that's still about twice the maximum possible density that our current theories would predict for objects at that scale.
So what's going on here? Astrophysicists aren't sure, but some of the explanations are pretty exotic. Dark energy could be a factor here - the assumption was that its distribution is uniform throughout the universe, but if it's positioned in certain areas it might be able to push together matter to form these vast structures. Another possibility is that our understanding of gravity is incomplete, and that we need something even more general than Einstein's general relativity to explain how gravity works at such scales.
That said, there's a simpler explanation that cannot be overlooked: this is all just a big mistake. There's no easy way to see something as big as hyperclusters, and our current observational techniques rely on some potentially error-prone techniques. If nothing else, astronomers always have to deal with having to see out of our own galaxy, and dust and stars inside the Milky Way can cloud our ability to see the rest of the universe clearly.
Efforts to confirm the existence of hyperclusters are currently underway, and we'll hopefully have a better sense of their existence in the next few years. If they do exist, then that would be a truly huge discovery, particularly if it forces us to expand Einstein's general relativity. And, if nothing else, the universe will have found yet another way to make us feel cosmically insignificant.