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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?

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    $\begingroup$ Wow, that statement from the youtube video has to qualify as one of the silliest I've heard in a long time. The issue you're talking about was the topic of the famous Shapley-Curtis debate of 1920: en.wikipedia.org/wiki/Great_Debate_%28astronomy%29 ... but what's 70 years among friends? $\endgroup$
    – user466
    Commented Jul 21, 2015 at 23:18
  • $\begingroup$ @Ben Crowell: Thoughts similar to yours were swirling through my head as I read the OP's question just now, since I read a lot of popular and semi-popular astronomy books when I was growing up (1960s, when the space race was in full swing). Your ending "but what's 70 years among friends" deserves more than one upvote, but that's all I can give. $\endgroup$
    – Dave L Renfro
    Commented Nov 30, 2016 at 16:56
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    $\begingroup$ Surely from Carl Sagan's series. $\endgroup$
    – Mikhail Katz
    Commented Jul 10, 2017 at 11:57

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That video comment is not precisely true in the sense that, the idea that we were one of billions of galaxies was not new to 1990 (quite the contrary), but it has an element of truth to it. Galaxies were known as "extragalactic-nebulae" in the 1920's. By 1929, when Hubble wrote his seminal work Extra-Galactic Nebulae we had already photographed several of them. Hubble's paper has many pictures. Here is a supplementary article (light reading), sketching the events from 1919 to Hubble's 1929 paper highlighting the issue, to wit, the universe is expanding. At the time, astronomers were identifying galaxies many millions of light years away.

Now the thing about the Hubble telescope is, it quite simply has allowed us to discover more galaxies. There is a nifty little article on hubblesite titled Taking the Universe's Baby Pictures that has some of these results. The first picture on that page claims to have about 10,000 galaxies in one image. That's the sort of thing made possible by the telescope. Here is another article, Hubble Discovers Oldest Known Galaxy . Again, a breakthrough made possible by the telescope. We can now see galaxies billions of light years away.

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    $\begingroup$ But was it merely an 'idea' (the way that say string theory today is an idea that is not universally accepted)? Or was there already a great deal of consensus, well before 1990, that our galaxy was merely one amongst billions? And if so, roughly when did that become the consensus? Shortly after 1929? Or much later than that? $\endgroup$
    – user67
    Commented Jul 21, 2015 at 23:50
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    $\begingroup$ I don't know when universal consensus among all scientists hit, even today we don't really know, but it is estimated that there on the order of 100 billion galaxies from what I can read. I guess its like any new science, someone like Hubble publishes game changing paper, many catch on the wagon; some just take a while. It was not like suddenly some astronomers looked at a telescope in disbelief in 1990. Amazement surely. We expected to see new galaxies. So I can't really tell you what every scientist was thinking in say 1950, but you can be sure the idea was no longer preposterous. $\endgroup$
    – J. W. Perry
    Commented Jul 21, 2015 at 23:56
  • $\begingroup$ The other opinion might be that there are an infinity of galaxies, but some number (100 billion?) describes from how many of them light can reach the earth, taking into account the speed of light and the time since the big bang. $\endgroup$
    – Gerald Edgar
    Commented Jul 10, 2017 at 13:02
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    $\begingroup$ @GeraldEdgar, since you seem to accept the big bang theory it would follow from the theory that the universe is finite ruling out the possibility that there are infinitely many galaxies and therefore also ruling out "the other opinion". $\endgroup$
    – Mikhail Katz
    Commented Jul 11, 2017 at 11:02
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    $\begingroup$ @MikhailKatz ... infinite universe is mathematically consistent with a "big bang" singularity $\endgroup$
    – Gerald Edgar
    Commented Jul 11, 2017 at 12:24

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