It's no surprise that galactic rotations do not follow Newtonian Dynamics, since galaxies host a wide variety of densities along their radius, so General Relativity and its time dilation effects should be taken into account.

Galactic cores are places where energy density is at its highest values, while galactic rims are places where energy density is almost that of outer space.

This means there's a "time gradient" along the radius of galaxies. There are places where time runs much faster (or slower) than in others. Time ticks much more slowly for observers near galactic cores than for those at the rims.

So for an observer near the core, the rims of all galaxies will appear to rotate much faster than just by the gravitational attraction of the baryonic matter it can detect, since its time units are bigger than those in regions where energy density is lower.

This observer will need a lot more Dark Matter to explain galactic rotation dynamics, since a great part of the speeds it observes for the rims come from this relative time difference. It will need a misterious agent to explain this velocities if it does not take gravitational time dilation effects into account. This observer lives in "slow motion" with respect to all other low energy density regions in the universe, thinking it's spacetime units can be applied to all other regions and phenomena it observes, when this is not the case.

On the other hand, an observer at the rim of a galaxy (in a low density environment) would need almost no Dark Matter content to explain the dynamics of the universe, since its time units are not subject to so much time dilation, so the speeds it observes will fit almost perfectly the baryonic matter content it can observe.