Dark Matter

The Universe is composed of 4% baryonic matter (real matter; protons, neutrons, electrons, etc), 73% dark energy, and 23% dark matter (1). Dark matter is estimated to make up 85% of the matter in the universe and the other 15% is made up of ordinary matter. Dark matter is referred to as dark because we cannot see it. Astronomers believe that there is hot, warm, and or cold dark matter. Dark matter is thought to be composed of non-baryonic and baryonic matter.

Astronomers hypothesize that hot dark matter is comprised of particles that have non-zero masses (2). The most popular hot dark matter particle is the neutrino. In 1930, Wolfgang Pauli first proposed the theory of neutrinos and in 1933, Enrico Fermi created the term Neutrino. Neutrinos are particles that are similar to electrons but they do not hold a negative charge like electrons, they hold no charge at all (3). Neutrinos can pass through great distances of matter and not be affected at all.   Since neutrinos are neutral, electromagnetic forces do not affect them (2). The only thing that can affect a neutrino is a weak sub-atomic force. There are three types of neutrinos; there is the electron neutrino, the muon, and the tau (3). Neutrinos also have what is called high-integer half spin.

Cold dark matter is theorized to be made up of WIMP (Weakly Interacting Massive Particles) and MACHOs (Massive Astrophysical Compact Halo Objects) (1). WIMPs are thought to be composed of massive Dirac neutrinos (4). The neutrinos that theoretically make up WIMPS are heavy fourth generation neutrinos. MACHOs are composed of normal baryonic matter that do not emit radiation. MACHOs could be black holes, brown dwarfs, neutron stars, planets, white dwarfs, and or extremely faint red dwarfs.

Although Dark matter cannot be seen it has been detected in many other ways. The first person to discover the presence of dark matter was Jan Oort. Oort, was a Dutch astronomer who studied radio astronomy. Oort discovered that the mass of galactic plane had to be more than what could be seen. When he discovered this he was studying stellar motions in surrounding of the local galactic area.

Dark matter can be detected when observing galactic rotational curves, the gravitational lensing of galaxy clusters, and by the velocity dispersions in galaxies (5). If we look at the Milky Way the way the stars in the arms move and the way the objects rotate around the Milky Way does not make since until we introduce more matter into the equation. If we only look at the visible matter in the Milky Way, the behavior of the objects inside the Milky Way do not make since. Gravitational lensing is the process by which space-time is curved by matter (5). The matter curves the space-time so light is then deflected. Gravitational lensing pertains to dark matter because non-visible matter has been confirmed to defect light (5). Dark matter also explains the high rotational speeds of galaxies (6). If these galaxies did not have the hidden dark matter they would be torn apart (6). 

1. National Aeronautics and Space Administration. Dark Energy, Dark matter. http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/
2. Dark Matter. http://csep10.phys.utk.edu/astr162/lect/cosmology/darkmatter.html
3. The neutrino and its friends. Dave Casper. http://www.ps.uci.edu/~superk/neutrino.html
4. WIMPs. Dave Spergel. March 6 1996. http://www.astro.princeton.edu/~dns/MAP/Bahcall/node8.html
5. Gravitational lensing. Jcohn. Berkley. Dec 13, 2010. http://astro.berkeley.edu/~jcohn/lens.html
6. Gravity Lens reveals dark matter. Bob Swarup. Aug 25, 2006. http://physicsworld.com/cws/article/news/2006/aug/25/gravity-lens-reveals-dark-matter