Why do most astrophysicists believe the cosmic microwave background (CMB) provides the best evidence for dark matter? What, exactly, IS that evidence?

Question

I frequently read that the cosmic microwave background contains the best overall evidence for the existence of dark matter, and conversely against alternative gravity theories like MOND.

However, I cannot imagine what that would be, given that dark matter is known for gravitationally influencing large galaxies and clusters of galaxies, and the CMB was released long before those things came into existence...

I am looking for a good description (technical or not) of the evidence for dark matter that exists in the CMB...