Caption: An animation of a computer simulation illustratin structure formation: (i.e., formation of the large-scale structure of the universe) over cosmic time ∼ 0.27--13.797 Gyr (i.e., lookback time ∼ 13.53--0 Gyr, where 0 is cosmic present = to the age of the observable universe = 13.797(23) Gyr (Planck 2018)) with cosmic time represented by cosmological redshift z ∼ 15--0.
The rotation of the computer simulation cube is rather annoying, but it allows you to see in multiple directions. The animation is also a bit for fast for easy comprehension.
Features:
Dark matter makes up 84.5 % of all matter according to best modern measurements (see Wikipedia: Dark matter: Overview).
Dark matter forms gravitational wells in which the ordinary matter clumps and forms visible structures form.
The galaxies form in what are called dark matter halos.
So there NO stars or galaxies in the simulation.
But you can imagine where they are from the clumping of the bright simulation particles.
Regions that are initially denser have runaway growth pulling in dark matter.
Regions that initially less dense lose dark matter and become voids.
In other words, "The rich get richer and the poor get poorer."
And that's how the cosmic web forms.
This means that (cosmological) proper distance size (true physical size) of the box increases with cosmic time due to the expansion of the universe, but that growth is "divided" out of the animation.
But it really happens in simulation.
a(t)/a_0 = 1/(1+z) ,where a_0 = a(t=present) = 1 by convention.
Since z goes from 30 to 0, cosmic scale factor a(t) a(t) increases from 1/31 ≅ 0.032258 ... to 1 or by a factor of 31.
Thus, the underlying cosmological model is probably the Λ-CDM model which is favored modern cosmological model.
The age of the universe according to the Λ-CDM model is 13.8 Gyr, and so the simulation probably starts within 1 Gyr of the Big Bang.
Probably, the simulation is met to represent structure formation starting from primordial density fluctuations.
This suggests that they are doing many things correctly.
However, significant disagreements or tensions between simulations and observations exist.
These might removed by improvements in the realism of the simulations.
However, it may be that key ingredients are missing from the simulations or that the Λ-CDM model is NOT the correct cosmological model of the observable universe.
Hopefully, the simulations will improve and give us more answers about the observable universe and the universe as whole which may be the multiverse.