Dark Matter is what we call the "stuff" that is causing the dramatic changes in motions of galaxy rotations and galaxy cluster speeds, as well as lensing effects.
Remember that when Newton presented his famous laws of gravity, when he was asked what gravity was, he admitted he didn't know. Being able to quantify its presence for given regions in the cosmos verifies its existence, and perhaps someday soon they will find what kind of particle, if such, it is.
Dark Energy is a label as well, but there is only one observable evidence for it, AFAIK, the acceleration of the universe. Einstein's cosmological constant is a math construct that existed before the acceleration discovery. Einstein tossed it, but others did not.