system
Given the following post, respond as if you're commenting on Reddit:
human
[astronomy] If the universe expanded at the same rate during the Big Bang why are there areas of high mass density( galaxies) and areas of low mass densities?
gpt
That is actually a great question, but very hard to answer. It touches on many different parts of cosmology and there is still a lot of debate around this.

One part is basically the question of initial conditions. What did the universe look like at the beginning? And why? Answering this involves topics like: string theory, inflation, pre-heating, baryogenisis, primordial nucleosynthesis. (In ascending order of how much we know about them). Some of this is very controversial and theoretical, so I'll leave it out.


What we do know *really* well is how the universe looked after those processes. At this point the universe is around 400 000 years old and filled basically with a neutral gas (mostly hydrogen). (We know this because of the Cosmic Mircowave Background). The gas is *almost* perfectly homogeneous, with fluctuations of about  10^(-5). Those initial fluctuations grow with time: denser areas have stronger gravitational pull, attract more gas and get even denser. Eventually they form galaxy clusters, galaxies and on the smallest scale gas clouds collapse to form stars. Therefor fluctuations are the seeds of the large scale structure we observe today. This is of course *way* more complex! Cosmologists use massive computer simulations to understand how exactly tiny fluctuations evolve into the universe we observe today. (There are many feedbacks/non-linear effects, dark matter...)

I hope this rough overview is somewhat helpful. Please ask if you want more details.