cold dark matter
When we say a gas is 'hot' or 'cold,' we really mean its particles move fast or slow. The same word is used for dark matter. 'Cold' dark matter means dark matter particles that move slowly compared to the speed of light — slow enough that, in the early universe, they could clump together under gravity rather than streaming freely across space. This single property turns out to shape the entire architecture of the cosmos.
Why does speed matter so much? In the young universe, matter was nearly smooth, with only tiny ripples in density. Gravity then amplified those ripples into galaxies and clusters. If dark matter were 'hot' — moving near light speed, like neutrinos — it would have raced out of the small ripples and erased them, so structure would have formed top-down, with giant superclusters fragmenting into galaxies. But the universe we see formed bottom-up: small clumps first, merging into bigger ones. That pattern only works if dark matter is cold and sluggish, able to hold on to small-scale lumps. Cold dark matter is the 'CDM' in the standard Lambda-CDM model.
Cold dark matter has been a triumph: computer simulations of a cold-dark-matter universe reproduce the cosmic web of filaments and voids, the clustering of galaxies, and the patterns in the cosmic microwave background with remarkable fidelity. It is not perfect, though. On the smallest scales there are nagging tensions — simulations predict more tiny satellite galaxies and denser galaxy cores than some observations show. These 'small-scale problems' may reflect messy ordinary-matter physics, or they may be hints that dark matter is slightly warmer or more complex than the simplest cold model.
Large cosmological simulations such as the Millennium Simulation start with a nearly smooth universe of cold dark matter and let gravity run for billions of years. They grow a tangled web of filaments, halos, and voids that matches the real distribution of galaxies in redshift surveys — strong support that dark matter is indeed cold.
Cold dark matter builds structure bottom-up: small clumps first, merging into the cosmic web.
'Cold' refers to particle speed (low velocity), not temperature in the everyday sense; a cold-dark-matter particle could in principle have a high mass while still moving slowly. Hot dark matter (like fast-moving neutrinos) is ruled out as the main component because it would have erased small-scale structure.