What you learned in school
Early 20th century astronomy textbooks taught that the Milky Way galaxy contained all matter and energy in existence. Students learned about "the universe" as synonymous with "our galaxy." Textbooks described the Milky Way as the complete cosmos, with spiral nebulae being mysterious objects within our galaxy.
What we know now
Early 20th century astronomy textbooks taught that our galaxy, the Milky Way, was the entire universe. Students learned about our galaxy as the complete cosmos, with nothing beyond it. The "Great Debate" in 1920 between astronomers about whether "spiral nebulae" were inside or outside our galaxy was a major scientific controversy. Edwin Hubble's observations in the 1920s proved that many "nebulae" were actually separate galaxies billions of light-years away. The universe contains hundreds of billions of galaxies, making our Milky Way just one of countless others.