What empirical relationship did Edwin Hubble formalize regarding distant galaxies in the late 1920s?

The farther away a galaxy was, the faster it seemed to be receding from the Milky Way.

Edwin Hubble's foundational work in the late 1920s involved systematic observation of distant galaxies. He discovered a direct proportionality between a galaxy's distance from us and its observed recessional velocity. This relationship is now codified as Hubble's Law. This discovery was revolutionary because it provided the first robust empirical evidence that the universe was not static but was, in fact, expanding. The observation that recession speed scales linearly with distance is the core characteristic that distinguishes cosmic expansion from simple outward motion from a single point.