“It’s the gravity that shapes the large scale structure of the universe, even though it is the weakest of four categories of forces.” -Stephen Hawking
Galaxies don’t just exist in isolation in our Universe, but are often found bound together as a part of even grander structures. Our own Milky Way is bound in a galactic group (our local group), nearby are larger groups and galaxy clusters, and on still larger scales, cosmic superclusters appear to encompass as many as 100,000 individual galaxies.
Outlined in light blue, giant collections of galaxies can be divided up into superclusters. But this classification doesn’t make superclusters real. Image credit: The Laniakea supercluster of galaxies R. Brent Tully, Hélène Courtois, Yehuda Hoffman & Daniel Pomarède, Nature 513, 71–73 (04 September 2014).
Yet it isn’t sufficient to simply see what appears to be a collection and draw an imaginary line around it. You can’t just give something a name and proclaim that it’s meaningful because you defined it. Instead, for a collection of objects in space, they need to be gravitationally bound together and connected. Thanks to dark energy, these superclusters aren’t.
A large collection of many thousands of galaxies makes up our nearby neighborhood within 100,000,000 light years. It’s dominated by the Virgo Cluster, but many other mass collections abound. Image credit: Wikimedia Commons user Andrew Z. Colvin.
Over billions of years, the galaxies in even our own Local Group will separate from the other clusters and groups nearby, and we’ll never wind up with a bound supercluster. Not here, and not anywhere in the Universe.