How do we know there are other galaxies?
Looking out at the night sky we can see many stars. But there are also other objects to be seen. In the 1920s astronomers confirmed that some of these objects were actually other galaxies.
As early as the 10th century, a Persian astronomer described the Andromeda galaxy as a “small cloud”. In 1920, the famous “Great Debate” took place. Two astronomers, Harlow Shapley and Heber Curtis, presented their arguments on whether fuzzy nebulae were quite small and within the outskirts of our home galaxy or whether they were in fact independent galaxies. Later that decade, Edwin Hubble observed the Andromeda Galaxy and measured the distance to some of its stars. He was able to confirm that they were 10 times further away than the furthest stars in the Milky Way, and concluded it was a separate galaxy.
Have you looked at the night sky? Mostly we see stars in the Milky Way galaxy. But, with telescopes, we can even see other galaxies!