Dark matter is a placeholder. It’s sort of a shorthand way of saying that there is more mass in the universe than what can presently be observed.
Like ‘gravity’, it’s just a word for something we don’t understand...............
Modern cosmologists invented dark matter to save the Standard Model, according to which, the universe should have flown apart by now.
Instead of questioning the fundamental validity of their underlying theories, they invent unobservable, unmeasureable phenomena that can't be checked or tested, to explain observations their theories have failed to predict.