But acceptance of the idea that there are galaxies out there is pretty new, no older than today’s oldest living citizens.
By the end of the 19th century, big telescopes had existed for over a century. The Herschel family (William, Caroline, John and Alexander) had so many, they sort of made a family business out of astronomy.
While the galaxies had been spotted, in more primitive telescopes they looked like fuzzy smudges and so they were thought to be nebulas … patches of dust and gas. (Like the famous ‘Horsehead Nebula’ in Orion.) And all the nebulas – glowing either by the stars within them or reflected starlight – were thought to be in “the universe” we now call the Milky Way.
But by the end of the 1800s, a storm was brewing between the “conservative” and the “liberal” astronomers. Then in 1908, a lady astronomer (rare indeed) named Leavitt discovered a way to calculate the distance to objects in space … by studying stars called Cepheids.
And in 1920 there was a Great Debate between members of the two factions (Harvard vs Pittsburg). In 1924, a man named Hubble wrote a paper that began to change everyone’s view of the Universe. He said that he had found Cepheids … stars … in the ‘great spiral nebula in Andromeda’. And that it is millions of light-years away. It is not a nebula. It is the Milky Way’s big brother. End of that debate!
In 1933 Swiss astronomer Fritz Zwicky was working at Caltech. He was studying a relatively nearby group of galaxies, over 1000 of them, called the Coma Cluster. (It’s about as wide in the sky as your thumb held at arm’s length.) He realized that they weren’t putting out enough light to explain their motions, and decided that much of them must be made of what he called ‘dark matter’.
The idea had to sit around waiting patiently for decades. (Am I suggesting that scientists will ignore hypotheses they personally don’t like? Yes! all the time.) Of course, the dawning realization that the universe was very, very large was keeping them all very busy at the time. Today, thanks to a telescope named Hubble, we know that there are more galaxies in the visible universe than there are stars in the Milky Way.
In the 1970s, lady astronomer Vera Rubin discovered that stars orbiting the outside of a spiral galaxy travel just as fast as those orbiting closer to the center. The question arose: why don’t galaxies fly apart? The answer hasn’t arrived!
Science moves on when confronted with enough facts. Of course, whether dark matter exists is still very much a question. Today, you could even say, there’s a great debate.