30th Day of December – Fatcowco – 1924 – Formal announcement of the existence of other galactic systems

At that time, the prevailing view of the cosmos was that the universe consisted entirely of the Milky Way Galaxy.

On the 30th December, 1924 Astronomer Edwin Hubble formally announces existence of other galactic systems at meeting of the American Astronomical Society

Source: 30th Day of December – Fatcowco