system
Given the following post, respond as if you're commenting on Reddit:
human
Why are super-clusters the largest objects in the observable Universe?

After recently watching a video about the Laniakea supercluster, a question arose about what prevents super-clusters from forming ever larger structures. Why for instance don't they form into spirals or discs made up of many super-clusters? Is it simply that the distances involved are too great? Any insights are much appreciated!
gpt
Essentially, because 1) the universe hasn't had time and 2) the universe is flying apart rather rapidly, which makes it harder for structure to grow indefinitely. 

If you had a static (meaning neither expanding nor contracting), infinite universe with a fairly smooth density distribution, then yes, you would expect structure to continue growing over time. However you would not expect gravitationally bound structures to have gotten very far beyond the supercluster scale, since the age of the universe is a pretty serious limiting factor. In reality, of course, our universe is not just expanding but expanding faster and faster, so it's getting pretty late in the game for large-scale structure to be really growing. Even the Great Attractor is actually receding from us. The matter in our supercluster will never all collect in the same place, we're receding too fast. The diagrams only represent the deviations in galaxies velocities from the Hubble Law (i.e., the change in their motion due to the tug from other matter). For most of our supercluster, the Great Attractor is only causing a moderate decrease in the rate at which the galaxies are flying apart.