If you looked into the night sky on a clear you would see the Milky Way. Millions of stars in our galaxy swirling in space like a giant spiral. Astronomers used to believe that this was the known universe. Everything was in our own Milky Way of stars. To believe otherwise was scientific heresy. There were those astronomers like Harlow Shapley that suggested that the fuzzy nebula were actually other universes well beyond our own Milky Way. Our star system was just a drop in the bucket compared to all the other universes out there in space. He was roundly ridiculed for his observations and theory. It would take decades of more observation and measurements before the scientific community eventually agreed with Shapely. Often in science it takes persistence and time to wear down old ideas and beliefs.
Read more about the Great Milky Way Debate at: https://www.sciencenews.org/article/astronomy-great-debate-island-universe-milky-way