What astronomical realization occurred primarily in the 1920s regarding faint, cloudy objects?

Answer

They were entire, separate island universes beyond the Milky Way

The confirmation in the 1920s established that the faint, cloudy objects seen through telescopes were, in fact, entire, separate island universes beyond the Milky Way.

What astronomical realization occurred primarily in the 1920s regarding faint, cloudy objects?
galaxyastronomyuniverseMilky Way