I suspect that if you asked most people, they could identify what a lot of basic astronomical objects are. Moons, planets, asteroids, stars. Fewer, maybe, could give you a good definition of galaxy — I see it misused a lot to mean solar system — but that's OK; it's not like you run across a galaxy every day (well, besides the fact that you live in one).
But that means that even fewer people know that galaxies tend not to go through the Universe solo. Lots of them live together in gigantic groups called clusters, and some of these clusters are big. Like, really big.
For example, meet Abell 2163.
Holy wow, that's gorgeous. Abell 2163 is not just any cluster of galaxies: It's one of the largest we know of. It may in fact be the largest … and it's possible it might be the single most massive structure in the entire Universe!
You can see hundreds of galaxies in this image, which was taken using the Hubble Space Telescope. And it doesn't even show the whole cluster! Abell 2163 is centered on the big fuzzy elliptical galaxy below center left, so quite a bit of it is outside this field of view.
Nearly every single object you can see here is a galaxy — I only count a couple of dozen obvious stars. You can spot the difference pretty easily: Compared to their physical size, stars are so far away they only appear as dots. Because of that, they have what are called diffraction spikes, those crosses of light splitting them. That's caused when the light from the star enters Hubble and bends around the thin metal strips holding its secondary mirror in place. The spikes are made from the light bending around the four vanes making up that structure (called the spider). Different color light bends differently, so the spikes are polychromatic.
Galaxies are much, farther away than stars, but also much, much, much bigger. So they appear as fuzzy blobs, not dots. They make diffraction spikes, too, but the light gets spread out, dimming the spikes a lot, so you don't see them (unless the galaxy has a very bright small core; that can make it look star-like in images and create spikes).
Anyway, my point is that the overwhelming majority of objects you see in this image are galaxies. It's hard to grasp: Each of these is a sprawling collection of billions or maybe even trillions of stars, massive amounts of gas and dust, and dark matter that surrounds and penetrates them. The scale of this is, quite simply, staggering.
Maybe I can help you understand this a little bit better. Instead of looking at the whole ginormous image, let's look at a piece of it. There's a nice little face-on spiral about a third of the way from the left and a quarter of the way down. See it? Let's zoom in on it:
Ah, there you go. I zoomed it a little more, too, and placed it as an inset on the right.
That is a galaxy: one of those immense cities of stars, gas, and dust. Moreover, it's a spiral galaxy, a flattened disk with arms winding around it. The arms aren’t stable structures; they’re more like gravitational traffic jams. All that material orbits the center of the galaxy in a disk, and complicated gravitational interactions can cause locations where material slows down when it enters that area and then resumes normal motion after it leaves. That's essentially what a traffic jam is, with cars slowing down as they move into it and speeding back up again as they exit it. In this case, though, the jam creates that huge spiral wave structure. Two of them, in this galaxy, with a third weaker one around the bottom right.
This is very much what our own Milky Way galaxy looks like. Oh, there are differences in detail, but overall our galaxy and this one are (very) distant cousins. It's smaller, though; measuring the size of the galaxy in that image, I find it's roughly half the size of the Milky Way.
Still, it's huge by human standards. It may be about 50,000 light-years across. That's a brain-freezing distance; a beam of light, the fastest thing in the Universe, takes 500 centuries to cross it. Written out, 50,000 light-years is 500,000,000,000,000,000 kilometers.
Another point: When you go outside on a very dark night, almost all the stars you see are pretty close in galactic terms; most are within a hundred light-years or so. Some powerhouses are visible from 1,000 light-years away. But even then, that’s only 1% of the distance across the Milky Way! The numbers of stars dotting the heavens seems enormous, but you're seeing only the nearest ones, a few thousand out of the several hundred billion in the galaxy.
Our galaxy is vast beyond our ability to tangibly grasp. Our brains choke on numbers like these.
Look again at that big image of Abell 2163. If you were to replace the spiral in it with our Milky Way, you'd barely be able to tell the difference. And in this image from Hubble, all that huge, soul-crushing galactic size is reduced to something a couple of dozen pixels across, its brilliance and excruciating detail also reduced to some faint, fuzzy smears.
And it's nearly lost among those other scattered denizens of the cluster. How can that be?
Abell 2163 is about 2.5 billion light-years away, a fifth of the way across the visible Universe, so far removed from Earth that you could block out the entire cluster holding up a pin head at arm's length. If a galaxy is enormous, the Universe is colossal. Its scale shrinks even the most ridiculously titanic of structures to grains of sand you can hold in your palm.
It's easy to be overwhelmed by this, to have your sense of ego crushed into dust. But remember: It took science, backed by math and engineering, to even know that these objects exist at all, let alone examine them, take their measure, and begin to understand them.
Yes, we humans are small on scales like this, but the fact that we even know about these scales broadens us. Our curiosity and imagination have made us huge.
There is so much to know, so much science to learn from observations such as this, but I am always heartened when I turn this around, and think about what it says about us.
And what it says is that when we are curious, we are mighty.