The mathematics describing this process are intractable except in very special cases. In most cases, physicists are obliged to use an approximation that produces an answer that they hope has much of the character of the correct answer. At one extreme, they just use Newtonian gravity, which produces almost-exactly correct answers for small-scale systems involving just regular stars and planets. It is only when warpage gets very large compared to the size of the system, or when e.g. galaxy-scale mass is involved, or the differences between Newtonian gravity and reality are what is interesting, that they have to resort to more complicated approximations.
It was recently discovered that calculations of the motion of galaxies were using an insufficiently accurate approximation that made it seem like stuff is orbiting too fast for the visible mass, requiring "dark matter"–extra, invisible mass–to hold the galaxy together. But using a more accurate approximation makes the need for dark matter evaporate. This created a problem because astrophysicists and cosmologists have come up with lots more uses for dark matter, to explain lots of other things. Without dark matter, they have dug themselves into a hole. The response has generally been to ignore the more accurate galactic gravitational model, and double down on dark matter. They can do this because ultimately it is all just a matter of papers being published and careers advanced or blighted; there are no other real-world consequences.