Dark matter makes up about 85% of the universe's total matter, dwarfing baryonic matter at just 15%. In the standard cosmological model, it drives galaxy rotation curves and the assembly of vast cosmic structures. It interacts primarily through gravity—so why doesn't it collapse into black holes?
In the universe's earliest moments, a quark-gluon plasma—a super-hot, dense soup of free quarks and gluons—dominated. Fractions of a second later, these particles combined into hadrons, forming the first atomic nuclei alongside electrons, neutrinos, photons, and dark matter.
These components evolved together, bound by gravity, but other forces shaped their paths. Photons and electrons clashed repeatedly via electromagnetic interactions, scattering and exchanging energy and momentum. Heavier atomic nuclei felt these effects less intensely.
Neutrinos interact only via gravity and the weak force, making collisions exceedingly rare. Dark matter takes isolation further: sensing only gravity, it avoids all collisions, gravitationally attracting—and being attracted to—baryonic matter without direct contact.
Related Reading: Primordial Black Holes Cannot Account for All Dark Matter
Collisions and interactions hinder baryonic matter from forming dense clumps and collapsing under gravity. Dark matter, free of these barriers, grows denser in cosmic regions—but differently from ordinary matter.
For baryonic gas, gravity compresses it, while electromagnetic bonds between atoms and molecules enable further densification, leading to stars, planets, and other compact objects.
Without electromagnetism for tight binding, dark matter forms only diffuse halos and filaments. Lacking inelastic collisions to shed energy, momentum, and angular momentum, even dense dark matter clusters remain extended structures. They sculpt the universe via gravity but cannot collapse into stars, planets, or black holes.
