How did dark matter shape galaxies and the cosmic web?

Why a web rather than an even spread?

Gravity amplified initial density differences. Cold dark matter could clump while ordinary matter was still strongly coupled to light. Later, ordinary gas fell toward those concentrations and could lose energy by emitting radiation. Sheets, filaments and dense intersections developed rather than a perfectly uniform distribution. ESA describes this formation history as the framework supporting stars and galaxies. ESA: structure formation.

Web is a description of this arrangement, not a claim that a material net stretches between galaxies. Its crowded regions and relatively empty spaces contain information about how matter moved under gravity. Seeing galaxies in a pattern also does not reveal the identity of the unseen particles.

Would the Milky Way exist without it?

Our galaxy's formation history depends on the dark component in the standard picture. NASA's January 2026 Webb report explains that dark-matter clumps helped ordinary matter collect and made galaxy formation begin earlier. Removing that component would change the history, so we should not expect the same Milky Way at the same time. That does not prove no galaxy-like system could exist under any different starting conditions or gravity theory. NASA's Webb formation report.

To answer a counterfactual precisely, researchers must say what else is held fixed. Are the initial fluctuations unchanged? Is gravity unchanged? Simply deleting an ingredient is not a complete description of an alternative universe. This is why a bounded statement about the standard model is more informative than an absolute claim.

What would particles bumping together mean?

Self-interacting dark matter would have appreciable scattering among dark particles, beyond their shared gravitational attraction. It need not involve solid little balls touching. Tulin, Yu and Zurek's original 2013 study models scattering through a light mediator and investigates its effects on halo structure. This is a physical proposal with testable consequences, not a confirmed particle detection. Original self-interaction model.

Why compare small galaxies and huge clusters?

A scattering probability can depend on relative speed. A model's effects may therefore differ between dwarf galaxies and much larger systems without using a different law in each. Sagunski and colleagues analyzed galaxy groups and clusters, finding constraints consistent with scattering that decreases at higher speeds. Their mild preference for nonzero scattering is not a conclusive discovery. Ordinary-matter effects and modeling uncertainties must also be considered. Original group-and-cluster analysis.

A useful explanation must work across these environments, not only one attractive example. Confidence is high in the broad gravitational formation picture; the microscopic scattering law is unknown. Robust cross-scale evidence distinguishing self-interactions from competing explanations would change that. Read how dark matter is inferred for the separate existence question.