That's dark energy. Dark matter would slow down the expansions of the universe, and they came up with that to explain the rotation curves of galaxies: they're

That's dark energy. Dark matter would slow down the expansions of the universe, and they came up with that to explain the rotation curves of galaxies: they're flat- all the stars in e.g. the Milky way go around the center in the same amount of time- but that shouldn't be the case going by just the matter we can see. Ergo, dark matter.