Some familiar materials that exist in the universe, they are made of neutrons, protons and electron and these materials are called the baryonic matter. The dark matter can either be made of non-baryonic matter or baryonic matter. To hold the universe elements together, the dark matter has to consist of about 80% of the matter (Bahcall et al. 2004).
One interest thing about the dark matter is that the scientists cannot see it. Then how do they know it exists in the first place? Scientists have learned to calculate the mass any huge object’s in the space through the study of their movement. In the 1950s when some scientists were examining spiral galaxies, they expected to witness the materials at the center to be moving fast as compared to those on the outside (Spooner and Kudryavtsev 2001). Against their expectations, the scientists discovered that the stars that were in the same position were traveling at a similar velocity. The results indicated that the galaxies are made of a lot of mass that cannot be seen. The elliptical galaxies studies further indicated that visible objects had more mass.
Albert Einstein proved that huge objects in the space bend light and distorted it, making them be used as lenses. Through the study of light distortion in the universe, the scientists have been able to create dark matter maps in the universe (Bertine 2010).
The gravitational mass can be derived from the observation of movement of the gas clouds and the stars motion is the disk as the spiral galaxy (for example the Milky way) in the disk as they circle the center (Rubin 1997). The galaxy rotation curve reveals how the speed of the stars around the center as the gap from the center increases. The majority of the spiral galaxies are known to reveal flat rotational curves out in the longest distance they can be traced, even after they disappear into the distance. It is, therefore, reasonable to