> Forgive me, but shouldn’t it be, “the majority of gravity” we observe?
Yes you're right, or maybe more precisely, "if we assume dark matter is a form of matter that behaves like normal mass in all ways except that it doesn't interact with light, then it must form the majority of mass in the universe."
> and dark matter theory continues because it makes the math work while it continues to fail experiment after experiment.
The point is that it doesn't fail experiment after experiment. If it had, it would no longer be a valid theory. It's annoyingly robust.
> We’re like a kid in school being asked to show their work on a math problem, knowing the answer, and tacking “+3” at the end because it came up short.
Sort of, but you make it sound like scientists are making things up rather than just doing experimental science.
We can explain dark matter if we modify the laws of gravity by some factor. What is that factor? OK, so how would modification of those fundamental laws affect other things in the universe? Can we see evidence of this happening anywhere, or not happening where we think it should? OK, we observed something that disproves that factor within a certain range of values. Does that give us more information, how can we test it more, does it give us insight into how the theory can be tweaked? Etc.