This quote from wikipedia is the accepted reason why it is said to exist. "Astrophysicists hypothesized dark matter due to discrepancies between the mass of large astronomical objects determined from their gravitational effects and the mass calculated from the "luminous matter" they contain: stars, gas, and dust."
First and foremost the scientists that decided it needed to be created to begin with had to first decide that their ability to calculate the masses of objects, (that are too far away to reliably gauge any relevant data except through speculation or guess work), somehow had to be irrefutable. Tell me first, how the data that was collected is irrefutable. You can't. Except in a land of non-science there isn't a need to create something to prove something else unless that something else is irrefutable. There isn't a way to calculate a stars mass exactly because we have absolutely no idea what is inside their cores and at best are making educated guesses based on what we know about our own planet. You might be right, but the data set containing the correct answer could possibly be anything. How does a logic minded person step from "we made a guess and the evidence says we are incorrect" to "obviously everything we have done is infallible therefore it must be something else we need to create in order to justify our previous GUESS".
I get it. People want this to be real. But there really isn't a justification for it's existence as a serious pursuit other than ego. There is no honesty in this approach. Honesty would be admitting that maybe we don't know as much as we claim about gravity and its effect on the fabric of space and even the tiniest degree off at the start will skew the end results dramatically. Honesty would be admitting that if we guessed wrong on the mass of stars, (and it is a guess no matter how educated a guess we claim), that any answer we get is somewhat suspect and not some infallible truth that demands creation of something else. You can take any wild guess or even an outright lie and through a long progression of made up supporting data make it appear to be fact. It doesn't make it a fact it just buries your original guess or miscalculation at the bottom of a giant pile of garbage.