That is the title of the YouTube video by Sabine Hossenfelder
Many points to respond to:
1) Scientists in Spain published a study in 2010, titled
M31's Odd Rotation Curve
Previously, there was a YouTube video of that title by one of the authors. I just checked and I could not find it, but maybe I missed it. The paper is still referenced by UniverseToday's web site.
I also found:
"Magnetic Fields and the Outer Rotation Curve of 31" in the Astrophysical Journal Letters
The conclusion was the galactic magnetic field explains the rotation curve. The paper implies they changed the model to use a specific value from M31 for their confirmation.
The galactic magnetic field would cause a relatively flat curve in the disk, as observed. M31 was expected to have its billions of stars follow orbits like our 8 planets around 1 star, our Sun. That assumption was unjustified. A complex disk having multiple spiral arms is nothing like our solar system.
2) Another alternative to dark matter was found in 2015.
An important conclusion after a study of IC342, a large obscured, nearby spiral galaxy:
Excerpt from the study from Max-Planck Institute titled:” Twisted magnetic field in galaxy IC 342"
"Spiral arms can hardly be formed by gravitational forces alone," continues Rainer Beck. "This new IC 342 image indicates that magnetic fields also play an important role in forming spiral arms."
3) Zwicky used invalid galaxy velocities. All galaxies have their velocity measured by atoms in the line of sight. That can never be the galaxy's velocity. For example, M31 has a blue shift because there are calcium ions in the line of sight and their absorption line blue shift comes from the ions, not M31, moving toward Earth. Dark energy is the excuse for wrong galactic velocities.
Dark matter is is the excuse for the mistake when atoms in motion are assumed to be the galaxy's motion.
4) Filaments are sometimes explained by dark matter. This ignores an important behavior of plasma in motion, where the magnetic field being generated maintains the filament. That behavior of filaments is why plasma has its name. There is no dark matter, but there is the unobserved magnetic field, just like in M31.
Dark matter is just assumed to be the reason for any missed magnetic field.
5) The right or wrong number of satellite galaxies is based on unfounded assumptions, not dark matter, which is explicitly in the name of the cosmological model to freely excuse what is not yet understood.