This is something you may not include in the story itself, but it’s still good to know. It’ll require you to do some extra deep worldbuilding, but it’s definitely worth it. A universe can certainly exist without knowledge of its origin (there isn’t too much knowledge about the origin of our own universe, aside from the Big Bang! But we don’t know as much about that as we would like to), but since you’re an author, and not a cosmologist trying to find out the origin of an existing universe, you’ll be able to dig deep enough to find the origin of your story’s universe, and how it evolved.
Every universe has some sort of origin, whether it exploded from nothing, or something like a god created it. You have quite a bit of leeway with how your universe is created, but as with everything, it should make sense within the context of the story. Here are some things to consider when thinking about a universe’s origin:
The multiverse might be unnecessary in your story, depending on the plot, but it’s still fascinating to think about. My story in particular involves inter-universe travel, so it was necessary that I design the multiverse as well, so I would know what I was doing when I wrote it. Sticking to just one universe, or even one planet, is definitely fine though.
Now you’ve got a universe, one that has just sprung into existence. What does the universe do now? It expands and evolves. The evolution of a universe pertains to many things. In the Big Bang model, matter and energy exploded from a single point, and as the universe’s vacuum expanded, so did the matter and energy. The atoms congregated into groups, forming clouds, then stars, and then planets.
As the matter becomes denser and forms planets, more complex molecules are also forming. These particles evolved and combined, giving rise to the most basic forms of life on Earth. If you are doing something that is based on Earth physics, it’s important to remember that there cannot be life without water. All life uses water. If you have another kind of life form that uses a different chemical, great! Just remember that it might not function well on Earth.
From the evolution of a universe comes the evolution of life, once it comes to exist. The most basic forms of life were probably proto-cells, tiny beads of liquid enclosed inside a membrane. Cells are the building blocks of life, so once these cells got more complex and gained organelles (basically organs inside a cell, but cell-sized), they started to form organisms with more than one cell. Unicellular life is still the most prevalent form of life on Earth, but it is what gave rise to multicellular life such as plants, animals, and fungi.
Basically, evolution means that organisms are becoming more complex (either that or changing). In terms of species, evolution happens when a species (not an individual organism) gains (usually) beneficial traits over time. For instance, a sparrow species could evolve to have more specialized beaks over time, because the sparrows with more specialized beaks originally would have survived more than the others, since those beaks were more efficient. For us on Earth, evolution eventually gave rise to a species that became aware of itself, the human. Because of this sentience, we were able to create societies and use tools. If you have a sentient species in your story (which I assume you do), they are probably one of the most evolutionarily advanced species on your planet. If you have more than one sentient species, conflicts could arise between them over who is the most “advanced” species, and that could add some conflict to your plot.
It's important to note that some traits that aren't as desirable (or neutral) get passed along, just because the individuals possessing them were the most successful at breeding. Orange fur in cats doesn't do anything in particular for cats, but if orange cats bred the most and were more successful for other reasons, orange fur would be more common as a staple of the species.
Sentient species also might take evolution to a morally questionable level, and implement something called “social Darwinism,” which is generally frowned upon these days. Social Darwinism is the idea that some races (or species, if you have more than one sentient species) are more advanced, and therefore superior. For instance, if you have a pure human species, they might look down upon humanoid species with more animalistic or plantlike features, even though those species might be just as advanced as they are (albeit possibly in different ways). Social Darwinism is a very problematic thing when it comes to everyday society, so it can definitely stir up some conflict in your story as well.
Evolution can also apply to technology. Since evolution is change over time, you might also want to document how technology developed throughout the society. I go over that in more detail in another article, but basically - you should figure out how they got from where they began, to where they are now. Stone tools to computers, I suppose. If they have gotten that far.
Here are some other things you can consider the evolution of:
Hopefully this is a good start when it comes to the origin of your universe and its evolution. Best of luck!