The Architecture of Everything
On the largest scales, the universe resembles a three-dimensional sponge. This structure, known as the cosmic web, is made of filaments, clusters, and enormous, nearly empty voids. The filaments are immense, thread-like formations of galaxies, gas, and dark matter that can stretch for 50 to 80 megaparsecs (160 to 260 million light-years). Some, like the Sloan Great Wall, are even larger, measuring nearly 1.5 billion light-years in length. Where these filaments intersect, they form dense nodes known as superclusters, which can contain thousands of galaxies. Our own Milky Way galaxy resides within the Laniakea Supercluster, a structure spanning 520 million light-years and containing the mass of 100,000 galaxies like our own.
The dominant component of this web is invisible. Dark matter, which makes up about 27% of the universe's content, is the underlying structure. baryonic matter—the stuff of stars and planets—accounts for only about 5% and is drawn by gravity into this dark matter framework. The vast regions between the filaments are cosmic voids, which make up about 80% of the universe's volume. The largest confirmed void, the Boötes Void, is a roughly spherical region about 330 million light-years in diameter that contains only about 60 known galaxies, whereas a typical volume of its size would be expected to contain thousands.
Seeing the Invisible Web
Astronomers map this immense structure not by seeing the web itself, but by plotting the positions of the galaxies within it. Large-scale galaxy redshift surveys, such as the Sloan Digital Sky Survey (SDSS), have created detailed 3D maps of the universe by measuring the distances to millions of galaxies. These maps clearly show the filamentary, web-like distribution of matter. The James Webb Space Telescope has extended this work, mapping the cosmic web back to when the universe was only a billion years old.
Another technique involves looking at the gas that exists between galaxies. This intergalactic medium (IGM) is not uniform; it traces the underlying cosmic web. By observing the light from extremely distant and bright objects like quasars, astronomers can see how it's absorbed by hydrogen gas in the IGM. This creates a pattern of absorption lines in the quasar's spectrum known as the Lyman-alpha forest, which shows the cosmic web's structure along the line of sight. The original seeds of this structure are visible in the Cosmic Microwave Background (CMB), the faint afterglow of the Big Bang. Tiny temperature fluctuations in the CMB, at a level of one part in 100,000, show the initial density variations that gravity amplified over 13.8 billion years to form the web we see today.