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What is Dark Matter? - A brief explanation

It seems to be more fashionable to support Weakly Interactive Massive Particles (WIMPs) rather than Modified Newtonian Dynamics (MOND) when explaining the Dark Matter phenomena. Dark Matter is the name given to the cause of the discrepancy between the classical prediction of galaxy rotation rates and the actual observations. The following image describes the difference between the two:

http://www.astronomy.ohio-state.edu/~thompson/162/Lecture40.html

The galaxies must have some additional –unseen– mass in order to explain these unpredicted rotation curves. This is basic conservation of angular momentum. Take the following simple example:

This really tiny galaxy has a mass of 100 pounds and an expected angular momentum of 1000 pound-feet per second. Its tangential angular momentum is given by L = mvr, so L (angular momentum) must be conserved. Plugging in the values, we get that v = 1000/(100*r) or v = 10/r. It becomes obvious that the speed should diminish if we increase the radius. Observations show that the speed remains fairly constant. The only factor that could maintain the conservation of angular momentum is some unaccounted mass.

The WIMP hypothesis attributes this mass to very heavy particles (10-1000 GeV range) that only interact through the gravitational and weak forces. On the other hand, MOND suggests that Newton’s law of gravity doesn’t hold for extremely small gravitational accelerations. It suggests that acceleration is not linearly proportional to force at these extreme values. Basically, it suggests that F does not! equal ma for very low accelerations.

Upon some research, I found some (sensationalist) articles that describe some oddities within the current theories:

http://hubblesite.org/newscenter/archive/releases/2012/10/full/

Recent observations suggest that Dark Matter isn’t necessarily bound to galaxies.

http://www.newscientist.com/article/dn20875-third-experiment-sees-hints-of-dark-matter.html

Here is some information about recent experiments regarding Dark Matter detection and the hypothetical particles that are thought to be WIMPs.