At first it was a simple idea that could account for a lot of observations. But then “observations have piled up that dark matter cannot explain”:
Citing a number reasons via the video and transcript. she says,
So, if it’s not dark matter then what else could it be? The alternative explanation to particle dark matter is modified gravity. The idea of modified gravity is that we are not missing a source for gravity, but that we have the law of gravity wrong.
Modified gravity solves all the riddles that I just told you about. There’s no friction, so high relative velocities are not a problem. It predicted the Tully-Fisher relation, it explains Renzo’s rule and satellite alignments, it removes the issue with density peaks in galactic cores, and solves the missing satellites problem.
But modified gravity does not do well with the cosmic microwave background and the early universe, and it has some issues with galaxy clusters.
So that looks like a battle between competing hypotheses, and that’s certainly how it’s been portrayed and how most physicists think about it.
Sabine Hossenfelder, “Dark Matter: The Situation Has Changed” at BackRe(Action)
She thinks the solution now is to combine dark matter with modified gravity. Now that will be a challenge. We kind of wish she would go back to denying free will and fine-tuning of the universe.