Universe Today
Glossary · Galaxies & Cosmology
The cosmic web
The largest structure there is: galaxies strung along filaments around enormous voids.
Matter did not spread evenly. Gravity amplified the tiny density variations left over from the early universe into a network of filaments and sheets, with clusters at the intersections and vast near-empty voids between. Galaxies form along the filaments and slide toward the nodes. Surveys mapping millions of galaxy positions — SDSS, DESI, Euclid — exist to measure the geometry of that web, because how fast it grew is the sharpest test of dark matter and dark energy available. The voids are as informative as the filaments. They are not quite empty — a few dwarf galaxies drift in them — and their sizes and shapes are set by the same physics as everything else, which makes them a clean test: dark energy stretches voids in a way that ordinary gravity alone would not, and they are large enough to measure that stretching directly.
Where it has come up lately
Related terms
Fast radio burst
A millisecond flash of radio energy from another galaxy, cause still unsettled.
Redshift
The stretching of light from distant objects, and the universe's odometer.
Hubble tension
Two good measurements of the universe's expansion rate that stubbornly disagree.
Dark energy
The name for whatever is making the expansion of the universe speed up.
Questions
What is cosmic web?
The largest structure there is: galaxies strung along filaments around enormous voids.
Why does it matter?
The voids are as informative as the filaments. They are not quite empty — a few dwarf galaxies drift in them — and their sizes and shapes are set by the same physics as everything else, which makes them a clean test: dark energy stretches voids in a way that ordinary gravity alone would not, and they are large enough to measure that stretching directly.