DARK MATTER

DARK MATTER: Dark matter is the nonluminous material distributed throughout the universe that cannot be directly detected by observing any form electromagnetic radiation but whose existence is suggested by gravitational effects on the visible matter. Dark matter is postulated to explain the flat rotation curves or spiral galaxies and other evidence of missing mass in the universe. According to present observations of structures larger than galaxies, as well as big bang cosmology, dark matter and dark energy account for the vast majority of the mass in the observable universe. The observed phenomena which imply the presence of dark matter include the rotational speeds of galaxies orbital velocities of galaxies in clusters, gravitational lensing of background objects by galaxy clusters such as the bullet cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies.

The galaxies near the milky way appears to be rotating faster than the rotation rate expected from the amount of visible matter that appears to be in these galaxies. Many astronomers believe that there is enough evidence to conclude that up to 96 percent of the matter in a typical galaxy is invisible. Many galaxies in the universe are grouped into clusters. Some astronomers argue that clusters galaxies are bound together from billions of years by the gravity due to the presence of enough mass which include upto 96% dark matter and energy (72% dark energy, 24% dark matter). According to inflationary big bang model the cosmological constant (Ω=ρ/ρc) describing the expansion of universe is close to 1. If the model is correct total mass of the universe must be about 100 times the amount of visible mass that appears to exists.

There are several possible candidates for the material that makes of dark matter. These include: neutrinos with mass, WIPS (weakly interacting massive particles, with are hypothetical subatomic particles far more massive than those produced in accelerator experiments), MACHOs (massive compact hello objects which include objects such as black holes that might form “hallo” around galaxies), undetected brown dwarfs (objects resembling stars, that are smaller and much fainter than the sun and are not parented by nuclear reactions) that interact with other particles only by gravity.