When the idea of dark matter first pushed its way into astronomers’ consciousness a few decades ago, the primary reaction was: “Seriously? There’s a mysterious, invisible substance out there, with a mass six or more times greater than that of the visible stars and galaxies, only we have no way of detecting it, but really, it’s there? OK then.” Or something like that, albeit in more formal scientific language.
These days, dark matter is a firmly established principle of cosmology; most of the questions now focus on how the stuff is distributed through the universe, and which of many possible subatomic particles it’s made of.
Most of the questions, but not all. Ever since the early 80’s, a competing theory has been struggling for acceptance.
Known as MOND, for Modified Newtonian Dynamics, it posits that dark matter’s main effect — allowing galaxies to spin faster than they should — isn’t caused by extra stuff, but instead by a change in how gravity works under certain conditions.