Many science-savvy people take it for granted that the universe is made not only of Carl Sagan’s oft-quoted “billions and billions” of galaxies, but also a vast amount of an invisible substance called dark matter. This odd matter is thought to be a new kind of subatomic particle that doesn’t interact via electromagnetism, nor the strong and weak nuclear forces. Dark matter is also supposed to be five times more prevalent in the universe than the ordinary matter of atoms.
However, the reality is that dark matter’s existence has not yet been proved. Dark matter is still a hypothesis, albeit a rather well-supported one. Any scientific theory has to make predictions, and if it’s right, then the measurements you do should line up with the predictions. The same goes for dark matter. For instance, dark matter theories make predictions for how fast galaxies are rotating. But, until now, measurements made of the detailed dark matter distribution at the center of low mass galaxies didn’t line up with those predictions.