Dark Matter

Form of matter that does not emit light, absorb light, or otherwise interact with electromagnetic radiation. Its only interactions are gravitational and dark matter particles can clump under the force of gravity (unlike “dark energy” which is evenly distributed throughout space). The existence of dark matter has been inferred from its gravitational effects on the dynamics of luminous tracers in galaxies and galaxy clusters and on the bending of light rays (gravitational lensing). Recent evidence indicates that dark matter contributes about 25% of the critical density for a flat universe, with the remainder, except ~5% ordinary matter, as dark energy. Much of the dark matter resides in halos encompassing luminous galaxies and in clusters of galaxies where it appears to be distributed more smoothly. Evidence from Big Bang Nucleosynthesis (BBN) and the cosmic microwave background radiation indicates that most dark matter is non-baryonic, that is, not composed of quarks. Particle physics theories predict the existence of exotic weakly interacting elementary particles (WIMPs), which are hypothetical candidates for non-baryonic dark matter. Dark matter plays a critical role in shaping the formation of large-scale structure. The bulk of baryons in the universe are also dark, perhaps in the form sub-stellar objects (MACHOs).

Some or all content above used with permission from J. H. Wittke.