In 1923, the universe was still thought to be a small place. Astronomers believed the Milky Way galaxy was at least tens of thousands light-years across, but they thought it was all there was to the universe — one grand galaxy spiraling in a lonesome ballet.
But … Hubble and Curtis hypothesized Andromeda was its own “island universe,” a self-contained system of stars, much like our own Milky Way. At the time, this was an outrageous idea, and was met with stiff, stubborn resistance by the scientific establishment. It would mean the universe was at least double its imagined size. Link