How would you program a model of a universe?

http://stars.chromeexperiments.com/

And I also had played this game on Steam like 10 years ago and loved it. Very basic, very simple. You started out as a small speck of dust in space and as you flew threw space, your gravity would begin to absorb the elements and you would get bigger, becoming a planet, a bigger planet, a sun, a black hole... while it was mostly simuluation and you had no real enemies, gravity is always our biggest enemy and you could be ripped apart by another black hole or even a giant sun, which made the game so enjoyable. You could drift and float and control it.. you flew through space and moved the arrow keys to avoid objects. You could change your speed too.

Let me see if i can find it and i only bring it up since I can't answer your exact answer, but can show you an idea. Looks like it got a massive upgrade as it used to be 2D. https://store.steampowered.com/app/230290/Universe_Sandbox/

And CodePen can at least get you started with a simulation.

https://codepen.io/search/pens?q=universe

Now, the direction and length of those vectors have to be continuously updated due to moving particles so that we can efficiently move particles arriving at any point.

The most logical explanation is that our universe updates its state at frequency that is directly linked to speed of light. This update frequency dictates the maximum distance any particle can travel in one cycle. Similarly gravity vectors cannot be updated faster than this frequency dictates. So gravity must propagate as spherical waves at the speed of light.

Similarly the system needs to continuously calculate electromagnetic field vectors for all points.

So when you place any fundamental particle in points X in the grid, it is immediately moved based on the direction and amplitude of the vectors in that point - but only by vectors that have been defined by rules to have an impact on this particular fundamental particle. We can e.g. define particles that are not effected by gravity at all.

So at very minimum we need some fundamental particles, forces and a way to calculate how those particles move in the grid. The system has an update frequency which defines the maximum speed in the grid.

Then you need to define rules what happens when two or more fundamental particles come close enough to each other.

When you have those rules and they produce an evolving system that stays in motion when fed with enough "input data", you have cracked this problem.

Solving this problem is the key to understanding theory of everything. So although the initial models aren't exactly correct, this exercise forces you to think like a God.