I discern it a little differently. I don't believe man can make a stable clock. I don't think a stable clock can be made from an oscillatory dynamic. The only stable ticker available is a rotation, not an oscillation. Our most sensitive sensors are oscillatory sensors because they are the most sensitive to external stimuli. We measure gravity, acceleration and orientation with oscillatory sensors. And unfortunately, we also measure time with atomic oscillations. Bad juju. Just asking for trouble.
A stable clock will not change with a change in elevation. Or gravity....or acceleration. The simplest clock is a rotating shaft. A rotating shaft has the same time all along the shaft. No matter the angle, gravity or acceleration.
Dark Matter.....it's not necessary yet. We can not measure the velocity of stars. Stars do not orbit in ellipses. As stars orbit, they spin. Stars are in helical orbits. And without knowing the radius of that spin, the length of the orbit remains un-known. Thus the velocity remains un-known. We can not measure this Z motion.
These helical orbits give the galaxy a wavy structure when viewed from the edge. This can give you an idea of that extra un-considered motion. They also spin as they orbit. That's more un-accounted length.
It would be very mis-leading to compare our solar system with a galaxy. The distribution of mass and gravity is much different. Solar systems have point gravity ratios, galaxies have shell gravity ratios. Star spins can vary greatly with radial position.
But even if one could measure this and get the correct velocity......you would still have great problems. Because gravity is not caused by "spacetime". Space and time are not physical. They do not have properties that can be varied. Only the properties of a physical entity can be varied.
Like light. All star light has a different velocity. We just don't know how to measure it. All we can measure is a self made reflection. A false measurement. And then base a whole science on it. Silly science.
But my discernments have always been contrary.