Let's try applying an analogy that should be familiar to all game developers.

The metric expansion of the Universe can be thought of as having a Scale transform applied to the Universe, and that Scale increases over time.

While that Scale is being applied and increased over time though, the speed of light continues to remain constant. Thus any two points in the Universe can be observed to be moving away from each other at a constant rate.

That Scale is only being applied at the macro level though, which is why we don't see red-shift from local light sources, such as our own sun or stars visible with the naked eye (and even stars visible with most amateur level telescopes, I believe). So the Scale gets applied to each Galactic component of the Universe, but those Galactic components don't apply the Scale to all their child components.

Now, if you're really trying to understand the WHY behind this apparent Scale transform being applied to the Universe, I think most (or perhaps all) theoretical physicists are still grappling with that one

You just shouldn't worry. We don't know nearly as much about the universe as one would believe. Unless you want to become astronomer, it probably won't make any difference to your life either.

All you really know is that some guy looked through his goggles and noticed a red shift. Although numerous credible scientists have since then confirmed that it's there, you likely haven't even seen it yourself. For all you know, it might not be true at all. But assume it's true that there is a red shift.

One possible explanation is that the universe is expanding. Probably that's the reason, too -- but you don't know. Another explanation would be that the universe has been expanding a million years ago, which is what we see, but it isn't expanding any more (maybe it's shrinking right now!). We only know what we see (and not what is true), and light travels at a quite finite speed whereas the universe is huge. So anything we see has already passed from history to legend to myth a long, long, long time ago.

Another explanation would be that the "vacuum" is not a vacuum but a very light red mist, so you only see its tint at astronomic depths (or incidentially there is no such mist in the solar system). Or maybe there exists mass that you can't see (something like a black hole, but weak enough so light can still escape) which the light has already travelled past, and that mass is now acting in the opposite direction. Maybe there is ... whatever. Maybe it's the Devil playing tricks on your eyes to test your faith. We cannot know, we can only tell what's plausible.

If you want to procedurally create a galaxy, create something that "looks like a galaxy" to you. Plausible, good enough. Who cares about the truth.

A simple (ish) technique

http://beltoforion.de/galaxy/galaxy_en.html

If you try to understand the universe, you will fail. For the simple reason that no one does (yet).

We have a very nice set of theories and equations that work MOST of the time, but we have problems. Data loss at black holes is probably the most famous I guess.

My favourite theory is that the universe is a simulation created by a computer programmer from a more advanced civilization.

The theory goes like this.....

We can create a simulation of the universe with modern super computers, but since we don't fully understand it and don't have enough computing power to make it real time, it's not real.

However we have seen a vast increase in computing power in the last few decades, so it is very highly probable that one day we will have the programming power to create a 100% accurate simulation.

We have also seen a vast increase in our understanding of the universe, so it is very highly probable that with enough time we will fully understand the universe.

Since it is highly probable we will be able to simulate the universe, and highly probable we will be able to understand the universe, it is highly probable that someone will create an artificial universe.

Since more than one programmer can create a simulation, if the above premise is true, then it is likely that multiple universes will be created.

Since there is only one real universe, and there are probably many more simulated universes, then it is more likely that we live in a simulation than in the real world.

Wrap your head around that one