In universe's first second, gravity and weak force formed baryon and non-baryon subatomic particles {dark matter}| that do not interact with electromagnetic radiation and move much slower than light speed {cold dark matter}. Because they do not interact with electromagnetic radiation, dark-matter particles are invisible. (Planets, cool gas, dust, and black holes interact with electromagnetic radiation and, though not visible, are not dark matter.)
Dark matter has mass. Dark matter is 23% of universe mass. Observed galaxy star (and gas) rotation rates show that dark-matter mass is nine times visible-matter mass.
Dark-matter particles have irregular mass differences. Because they do not have smooth small-scale mass differences, dark matter is not subatomic particles that move at near light speed {hot dark matter}.
About 900 million years after universe origin, 12.8 billion years ago, dark matter formed clouds, with masses million times Sun mass. From then until 7 billion years ago, clouds contracted and merged and made evenly spaced spherical clouds, with masses five to ten times more than visible and non-visible ordinary matter there. These clouds have enough matter to make gravity form galaxies. If matter was only ordinary matter, density is not enough to let gravity form galaxies. Because galaxies have five to ten times smaller volume than dark-matter clouds, dark-matter density and ordinary-matter density are approximately equal.
Because they have mass, dark-matter particles exchange potential and kinetic energy with gravitational fields, which change as particles move, so dark-matter particles and gravitation fields settle into virial equilibrium.
Dark-matter particles never collide.
Universe matter distribution is more even than expected by particle statistics {smoothness problem}.
Perhaps, real particles {ylem} can come from virtual-particle pairs, making universe more homogeneous.
Perhaps, new subatomic-particle types {ghost condensate} can make outward pressure that prevents gravitational collapse.
Perhaps, cold-dark-matter particles are superpartner particles {lightest superpartner particle} (LSP).
Perhaps, dark matter is neutrinos or supersymmetric particles {photino} {Zino} {Higgsino}.
Perhaps, cold dark matter is subatomic particles {weakly interacting massive particles} (WIMP) {neutralino} from universe origin that do not interact with electromagnetic radiation and move slowly.
5-Astronomy-Universe-Cosmology
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Date Modified: 2022.0225