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.
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