What term describes the established contradiction between theoretical Keplerian predictions and observed flat rotation curves?

The galactic rotation problem.

The term 'galactic rotation problem' is specifically assigned to the profound contradiction observed when comparing theoretical expectations of stellar orbital speeds in spiral galaxies against actual astronomical measurements. The theory, relying on visible matter and Newtonian gravity, predicts that velocities must fall off according to Keplerian dynamics ($v o 0$ as $R o ext{infinity}$). However, observations consistently show that these velocities plateau or remain high even at the galaxy's outermost edges. This stark, repeatable failure of established gravitational theory based on luminous mass to explain observed motion is collectively termed the galactic rotation problem, making it a central puzzle in modern astrophysics.