Darwin's theory of evolution is descent with modification. It requires a method of inheritance, a method of random variation, and the observation that a variation which results in more descendants will grow exponentially relative to the rest of the population, until it dominates.
Examples of specific theories include the Modern Synthesis and Punctuated Equilibrium. But there are more specific theories as well. For example https://www.amazon.com/Ontogeny-Phylogeny-Stephen-Jay-Gould/... lays out evidence that one of the major drivers of evolution is that it is easier to change the relative timing of maturation, but almost impossible to change the sequencing. And we have specific examples of evolution where the method of modification is not mutation. For example the placenta is believed to be the result of viruses transfering a parasite genome into the mammal genoume.
The theory of dark matter is the theory that a large fraction of the matter that makes up galaxies is not visible.
You can get different theories based on how much of it we think that there is, and what we think that it is made of. There isn't a ton of room for how much of it there is - Newton plus gravity curves gives us that. But the what is fascinating. For example you might think that it is mostly things like wandering planets and stars that did not ignite. This gives us the theory of MACHOs. But we've been able to establish that most of it can't be that. We've also managed to establish that not a single thing in our known particle zoo will work. And so it has to be something of a kind we've yet to discover.
And that's where we have the challenge. We start with the idea that there's a lot of stuff that isn't shining stars. That's very reasonable. Then we wind up with the conclusion that our galaxy is mostly stuff of a kind that physics knows absolutely nothing about. That's...substantially more surprising.