system
Given the following post, respond as if you're commenting on Reddit:
human
Why do galaxies form superclusters throughout the universe instead of being evenly or randomly distributed in space?

Every model of the universe shows "webs" of galactic clusters with gaping voids between them (Boötes void being the most famous of them). But on a universal scale, galaxies are tiny, so I couldn't imagine that they're so dense that they'd have that amount of attraction to each other across *gigaparsecs* of distance.

Much in the same way that diffuse galaxies have no apparent shape (despite often having huge amounts of stars), I'd think that the same would occur with the universe as a whole.

Or maybe physics works differently at such massive scales?
gpt
Okay lemme paint you the picture. The Universe starts very hot, super compact, with tiny and random quantum fluctuations in the density of the matter as things jiggle back and forth. Then an event called 'inflation' occurs. The Universe very vary rapidly expands by many orders of magnitude. All of a sudden, those density fluctuations get blown up massively. They're now huge, and very far apart, so they can't reach equilibrium too easily.

The leftover heat (with a little help from gravity) sets up oscillating waves of density of different sizes all across the Universe. These are called the Baryon-Acoustic Oscillations (they really were acoustic! Moved just like sound waves!) As it continues to cool, certain sizes of oscillations are selected for and then 'frozen in' (they cool enough to stop oscillating), and this sets in a pattern of structure in the form of density fluctuations. Overdensities become the birthplace of stars. Underdensities become voids in which nothing happens.

It is important to note that dark matter does not oscillate, since as far as we know it exerts no pressure and can collapse freely. This dark matter collapse predates and predicates the baryonic collapse. Without dark matter determining where the baryons oscillate, there would be no structure!

There are now two forces at work. Gravity wants to bring things together. Dark energy pushes them apart by expanding voids. In overdensities, gravity wins, and stars form galaxies; galaxies form clusters; clusters form filaments. In underdensities, dark energy wins and pushes the clusters further away from each other. This creates the web-like structure we see today.