Editor's note: A version of this was previously published on the author's website.
Until the early 1920s when the astronomer Edwin Hubble proved that the neighboring great galaxy in Andromeda is actually a separate galaxy, our Milky Way was the universe — or so almost everybody thought. What we now know as galaxies were believed by some scientists to be structures like nebulas, gas clouds such as the Orion Nebula. One of the better guesses about the universe's (our galaxy's) size was 300,000 light-years across.
But in 1923, making and examining glass negative photos at the Mount Wilson observatory in California, Hubble detected a variable star in the Andromeda "spiral nebula" — a Cepheid variable, a variety of known size and luminosity that dims and brightens over a predictable period. By the end of the next year, he had found 36 variable stars in Andromeda, 12 proving to be Cepheids.
Scientists had been able to work out the distance to Cepheid variable stars near Earth by using a spectroscopic method, according to Earth-Sky.org. Then, with a star's known magnitude as a marker, the distance to a Cepheid in the way-beyond could be figured — the fainter, the farther away. Using the luminosity of the Andromeda galaxy's Cepheids, Hubble calculated its distance as 900,000 light-years. (Modern measurements place it at 2.4 million light-years.) Undeniably, Andromeda was an "island universe" separate from our own, with all the nebulas, planets, millions of stars and other accouterments of a well-dressed galaxy. Therefore, many of the other supposed nebulas also were galaxies
Since the early 1920s, our understanding of the universe has expanded to include billions of galaxies, some of them billions of light-years away.
Once the nature of galaxies was determined, it became clear that most are members of galaxy clusters.
"Galaxies can swarm together to form groups and clusters of galaxies held together by their mutual galaxy," write Harvard University experts with NASA's Chandra X-Ray observatory. "X-ray observations show that these enormous systems of galaxies are filled with colossal clouds of hot gas. These clouds have temperatures as high as a hundred million degrees and contain as much mass as all the stars in the galaxies in the cluster."
NASA points out that about three-quarters of all galaxies may belong to clusters. Clusters may number "from afew to a few thousand galaxies bunched together and moving around within their combined gravitational field," notes the University of Turku, Finland.
Some galaxy clusters are visible through small telescopes; our own Milky Way is part of the Local Group of galaxies, about 30 island universes that include the Andromeda Galaxy (M31); Andromeda's satellite galaxies M110 and M32; and the Triangulum Galaxy, M33. Andromeda is the closest, while M33 is about 3 million light-years away.
In addition, clusters of galaxies can belong to what is termed superclusters, which are groupings of galaxy clusters that are somewhat adjacent "but not necessarily gravitationally bound," the Harvard site explains.
"It is the proximity of some 50 nearby small groups of galaxies to the Virgo cluster that suggests that they all form an enormous flattened cluster of clusters; we call it the Local Supercluster," says a NASA site. Exceeding these monstrously big structures are what are known as "walls" of galaxies, which are streams of clusters and superclusters.
The largest known, called the BOSS Great Wall (BOSS stands for the Baryon Oscillation Spectroscopic Survey, a project that discovered the object), is close to 6.5 billion light-years away and contains 830 galaxies, according to a 2016 research letter published in the journal Astronomy & Astrophysics. One of the authors of the paper, "Discovery of a massive supercluster system at Z ∼ 0.47," is Antonio D. Montero-Dorta, who at the time was with the University of Utah Department of Physics and Astronomy in Salt Lake City.
A favorite with amateur astronomers is a galaxy collection that isn't a cluster. Dubbed the Deer Lick Group, it only looks like a cluster because its line of sight from Earth encompasses several galaxies. It contains one relatively close galaxy and a few that are much farther away.