The ellipse that defines the orbit of Mercury around the Sun, itself rotates (precesses) and causes the longitude of ascending node, longitudes of descending node, and point of apoapsis of the orbit to advance a bit with each orbit. This is because the Sun is not a perfect sphere, but has a bulge around its equator due to its rotation. This in turn creates a non-uniform gravitational field, described by general relativity, that causes the precession of Mercury's orbit. Similarly, the Earth is also not a perfect sphere, it is slightly flattened, oblate spheroid, due to its rotation, by a factor of 1/299. General relativity is used to design the orbits of Sun-synchronous satellites, which precess a little and keep the plane of the orbit always oriented somewhat perpendicular to the Sun-Earth radial line, and maintains continuous exposure of the satellite to the Sun. By extension, an object like the Milky Way galaxy, which is as flat as a pancake, ought to have significant relativistic effects on the orbits of stars. The angular velocity contribution from the precession of orbits might give stars enough apparent tangential velocity to make it seem like the galaxy has a lot more mass than can be explained by a Newtonian gravity model. It's one obvious example of why Newtonian gravity should not be used for cosmological models. If it doesn't work to explain the orbits of Earth-orbiting satellites and planets, why would they use it for galaxies?!? Since the introduction of relativistic effects has eliminated the need for dark matter to explain the velocity curves for at least four galaxies, including our own, I expect that dark matter will explain the velocity curves of every other galaxy as well, and be debunked in the very near future. In fact, I can't believe they didn't do this decades ago. General relativity is not a new theory, published in 1915.

https://en.wikipedia.org/wiki/General_relativity