Why dark matter?

Why is dark matter necessary?

I understand (from the media) that dark matter is postulated as the force that causes (among other effects) stars on the outer spirals of galaxies to orbit at much higher speeds than the mass contained in these galaxies imply that they should. I understand dark energy is postulated as sort of an “anti-gravity” force emanating from a vacuum that causes the universe to expand and accelerate faster than it should based on the amount of available energy.

So why do you need both? Couldn’t a “push” from dark energy accelerate the stars on the outer spirals? How would they know if they were being pulled by dark matter or pushed by dark energy?

DGR

Dark energy would be far too weak to noticeably affect an area as small as a galaxy in that fashion. It also pervades the entire fabric of space equally, so the way a galaxy rotates wouldn’t vary the same way. And an antigravity push doesn’t describe the effect that dark matter has in any way.

Thanks for the explanation!

DGR