It does seem like that, and so there have been many attempts to revise the math to make it fit all known observations. But nobody has been successful, and so dark matter remains, at the very least, by far the best explanation we have. It fits the data.
To use an analogy from the world of finance, that sounds like building a Balance Sheet model that "plugs" Retained Earnings so that Assets and Liabilities always balance. Of course it fits the data, its very existence is to explain away the part of the model that doesn't fit the data
Dark matter is not just an imaginary number that fills some holes in math. It is a thing that has been observed. Galaxies can have it or not have it. Galaxies can be separated from it in collisions. You can't be separated from something that doesn't exist.
This article has introduced me to "Modified Newtonian dynamics", which is a field of study I didn't know existed. The premise is pretty obvious now: Newton was pretty cool and all, but he wasn't right about high acceleration environments. Why then should we presume that he was be right about low acceleration environments, when the observations don't fit the predictions? The fact that nobody has come up with an answer doesn't mean there isn't any, and at least these people are actively trying to come up with one. I understand why the author appears to be very frustrated -- I'd be frothing-at-my-mouth angry in his position.
