This Kurzgesagt YouTube video claims that
Without [the Hubble Space Telescope], we would never have known that our galaxy is one of billions in an enormous universe. But, the actual human, Mr. Hubble, discovered another galaxy in 1929 (Andromeda).
I found this claim surprising (note: I do not have much more than high school physics). According to Wikipedia, the Hubble Space Telescope was launched only in 1990. I would have thought that we already knew that there were very many other galaxies, long before 1990.
So, when did scientists know (with a great deal of confidence) that our galaxy was merely one amongst billions in the universe?