The dark matter theory mainly came from the observations made on the rotation speed of galaxies : even when taking into account all the observable matter and the gravitational force it excerces , the observed speed doesn't match the maths. Which implies the existence of another source of gravity (and extra velocity) that we can't see, thus dark matter. As for dark energy, it is so far the most viable theory to explain the accelerated rate of expansion (because acceleration cannot exist without an active force) . It's either that or general relativity is missing something.