They’re comparable in size to the most supermassive black hole event horizons.
This diagram shows the relative sizes of the event horizons of the two supermassive black holes orbiting one another in the OJ 287 system. The larger one, of ~18 billion solar masses, is 12 times the size of Neptune’s orbit; the smaller, of 150 million solar masses, is about the size of the asteroid Ceres’s orbit around the Sun. There are precious few galaxies, all much smaller than our own, that have a supermassive black hole of “only” ~4 million solar masses. (Credit: NASA/JPL-Caltech/R. Hurt (IPAC))
But even the largest individual objects are no match for cosmic collections of objects.
The Solar System, as viewed on a logarithmic scale, highlights just how far away some of the objects are. The planets, the Kuiper belt, the Oort cloud, and the nearest star are all shown here, with Voyager 1, presently 155.5 AU from the Sun, our most distant artificial spacecraft. (Credit: NASA/JPL-Caltech)
Around each stellar system, Oort clouds span multiple light-years: tens of trillions of kilometers.
An illustration of the inner and outer Oort Cloud surrounding our Sun. While the inner Oort Cloud is torus-shaped, the outer Oort Cloud is spherical. The true extent of the outer Oort Cloud may be under 1 light-year, or greater than 3 light-years; there is a tremendous uncertainty here. Comet Bernardinelli-Bernstein has an aphelion of just under 1 light-year, suggesting that the Oort cloud is at least that large. (Credit: Pablo Carlos Budassi/Wikimedia Commons)
The stars themselves cluster together into great galactic assemblages.
Only approximately 1000 stars are present in the entirety of dwarf galaxies Segue 1 and Segue 3, which has a gravitational mass of 600,000 Suns. The stars making up the dwarf satellite Segue 1 are circled here. As we discover smaller, fainter galaxies with fewer numbers of stars, we begin to recognize just how common these small galaxies are; there may be as many as 100 in our Local Group alone. (Credit: Marla Geha/Keck Observatory)
At minimum, they possess thousands of stars, spanning hundreds of light-years.
The giant galaxy cluster, Abell 2029, houses galaxy IC 1101 at its core. At 5.5-to-6.0 million light-years across, over 100 trillion stars and the mass of nearly a quadrillion suns, it’s the largest known galaxy of all by many metrics. It’s unfortunately difficult for the Universe to make a single object significantly larger owing to its finite age and the presence of dark energy. (Credit: Digitized Sky Survey 2; NASA)
In a first-of-its-kind image, the scale of galaxies, including the Milky Way, Andromeda, the largest spiral (UGC 2885), the largest elliptical (IC 1101), and the largest radio galaxy, Alcyoneus, are all shown together and, accurately, to scale. (Credit: E. Siegel)
On even larger scales, galaxies cluster together, forming structures up to hundreds of millions of light-years across.
The impressively huge galaxy cluster MACS J1149.5+223, whose light took over 5 billion years to reach us, is among the largest bound structures in all the Universe. On larger scales, nearby galaxies, groups, and clusters may appear to be associated with it, but are being driven apart from this cluster due to dark energy; superclusters are only apparent structures, but the largest galaxy clusters that are bound can still reach hundreds of millions, and perhaps even a billion, light-years in extent. (Credit: NASA, ESA, and S. Rodney (JHU) and the FrontierSN team; T. Treu (UCLA), P. Kelly (UC Berkeley), and the GLASS team; J. Lotz (STScI) and the Frontier Fields team; M. Postman (STScI) and the CLASH team; and Z. Levay (STScI))
The Sloan Great Wall is one of the largest apparent, thought likely transient, structures in the Universe, at some 1.37 billion light-years across. It may just be a chance alignment of multiple superclusters, but it’s definitely not a single, gravitationally bound structure. The galaxies of the Sloan Great Wall are depicted at right. (Credit: Willem Schaap (L); Pablo Carlos Budassi (R)/Wikimedia Commons)
The size of our visible Universe (yellow), along with the amount we can reach (magenta) if we left, today, on a journey at the speed of light. The limit of the visible Universe is 46.1 billion light-years, as that’s the limit of how far away an object that emitted light that would just be reaching us today would be after expanding away from us for 13.8 billion years. There are an estimated 2 trillion galaxies contained within the yellow sphere drawn here, but that estimate is likely low, perhaps by as much as a factor of 3-to-10. (Credit: Andrew Z. Colvin and Frederic Michel, Wikimedia Commons; Annotations: E. Siegel)
This simulation shows the cosmic web of dark matter and the large-scale structure it forms. Normal matter is present, but is only 1/6th of the total matter. Meanwhile, matter itself only composes about 2/3rds of the entire Universe, with dark energy making up the rest. The unobservable Universe must extend for at least ~400 times the extent of the visible Universe we can see, meaning that our 92 billion light-year diameter Universe is less than one-64-millionth of the minimum volume of what’s out there. (Credit: The Millennium Simulation, V. Springel et al.)
While many independent Universes are predicted to be created in an inflating spacetime, inflation never ends everywhere at once, but rather only in distinct, independent areas separated by space that continues to inflate. This is where the scientific motivation for a Multiverse comes from, why no two Universes will ever collide, and why we fully expect the unobservable Universe to tend towards infinite size as time goes on. (Credit: MUSTAFABULENT / Adobe Stock)