While this may not seem obvious, galaxies are not just randomly distributed throughout the universe. Instead, they are grouped into large strands separated by giant voids of space. Each strand is basically a wall of galaxies spanning hundreds of millions of light years. Interestingly, one of the largest structures in the known universe was discovered by astronomers quite recently, and this is a giant wall of galaxies about 1.4 billion light years long! Given how close this massive structure is to us, it's surprising that scientists didn't notice it before. Over the past ten years, an international team of astronomers led by Brent Tully of the University of Hawaii's Institute of Astronomy has been mapping the distribution of galaxies around the Milky Way. Astronomers have named this newly identified structure the "South Pole Wall," which sits outside Laniakea, a huge supercluster of galaxies, including our own.
Universe on a large scale
On its largest scale, the universe looks like a huge cosmic web. Stars connect to form galaxies, which are grouped into galactic groups. Many groups tied together lead to clusters of galaxies, and sometimes clusters merge together to create even larger clusters. The many clusters together, spanning hundreds of millions or even billions of light-years across, appear to form the largest structures of all: superclusters.
Our own supercluster, Laniakeya, is made up of roughly 100,000 galaxies, more than 10 times richer than the largest known clusters. However, these superclusters only appear to be structures. As the Universe ages, individual components of superclusters move apart, showing that they are not true structures after all.
Laniakeya and the neighboring Perseus-Pisces supercluster of galaxies. Image: nature.com
The hot sea of matter and radiation, being dense and expanding, cools over time. As a result, atomic nuclei, neutral atoms and, ultimately, stars, galaxies and their clusters will be formed over a sufficiently long time. The irresistible force of gravity makes this inevitable, thanks to its effect both on ordinary (atomic) matter, which we know, and on the dark matter that fills our Universe, the nature of which is still unknown.
Beyond the Milky Way
When we look out into the universe - beyond our galaxy, this picture is of great importance. At least it seems so at first glance. While many galaxies exist in isolation or are grouped into collections of only a few, there are also huge gravitational wells in the universe that pull hundreds or even thousands of galaxies, creating huge clusters.
Quite often, there are supermassive elliptical galaxies in the center, with the most massive detected so far is shown below: IC 1101, which is more than a thousand times more massive than our own Milky Way.
The most massive galaxy known - IC 1101 - looks like this.
So which is larger than a cluster of galaxies? Superclusters are clusters of clusters connected by large cosmic filaments of dark and normal matter, the gravity of which mutually attracts them to their common center of mass. You wouldn't be alone if you thought it was just a matter of time - that is, time and gravity - when all the clusters that make up the supercluster merge together.When this happens, we will eventually be able to observe a single coherent cosmic structure of unprecedented mass.
Local group of galaxies
In our own region, a local group consisting of Andromeda, the Milky Way, the Triangle and possibly 50 smaller dwarf galaxies sits on the outskirts of the Laniakea supercluster. Our location puts us roughly 50,000,000 light-years from our main source of mass: the massive Virgo Cluster, which contains over a thousand galaxies the size of the Milky Way. Many other galaxies, galaxy groups and small clusters can be found along the way.
On an even larger scale, the Virgo Cluster is just one of many in the part of the universe we have mapped, along with the two closest ones: the Centaurus Cluster and the Perseus-Pisces Cluster. Where galaxies are most concentrated, they are the largest clusters of mass; where lines connect them along the threads, we find "threads" of galaxies, similar to pearls, too thin for a necklace; and in the big bubbles between the filaments we find an immense lack of density of matter, as these regions have given up their mass to the denser ones.
The Milky Way is surrounded by other, smaller galaxies.
If we look at our own environment, we find that there is a large collection of over 3,000 galaxies that make up a large-scale structure that includes us, Virgo, Leo, and many other surrounding groups. The dense Virgo Cluster is the largest part of it, accounting for just over a third of the total mass, but it contains many other concentrations of mass, including our own local group, bound together by invisible gravity and invisible threads of dark matter.
Great, isn't it? But in fact, these structures are not real. They are not related to each other and will never become so. However, the very idea of the existence of superclusters and the name for ours - Laniakei - will persist for a long time. But just by calling an object, you cannot make it real: in billions of years, all the various components will simply be scattered farther and farther from each other, and in the most distant future of our imagination they will disappear from the field of vision. All this is due to the simple fact that superclusters, despite their names, are not structures at all, but simply temporary configurations that are destined to be torn apart by the expansion of the Universe.