Dark Matter is a great topic of interest today for cosmologists. No one yet knows what this entity is and, frankly, few have any clue. Some believe that it is made up of virtual particles, and some envisage a universe filled with gravitons, which are hypothesized particles that transmit the force of gravity. In either case, dark matter makes up roughly 90% of the total mass in the universe and is therefore necessary to understand.
Many questions have been asked about whether dark matter really exists. After all, how could we know that something exists if we do not know what it is and don't even have the slightest clue? The first evidence of dark matter presence came from the observation of galaxies. Scientists measured the speed of the matter from the center outward. It was found that the speeds of even the most distant visible objects were very similar to the objects towards the center. This implies that there must be a tremendous amount of mass other than what is visible. This is because the orbital speeds of the matter in the galaxy give us a clue as to how much matter must be inside a galaxy. The speeds of the orbiting matter were determined by measuring the red and blue shifts. As the orbiting matter speeds towards us, a slight blue shift is created; as it speeds away from as, a slight red shift becomes apparent.

Through a series of calculated formulas, it was discovered that, for example, the Milky Way Galaxy contained 90 billion solar masses. Yet, the luminosity of the galaxy is equal to merely 15 billion solar luminosities, indicating a mass to light ration of 6-1. As scientists measure the mass of the outer regions it becomes apparent that much mass is present, yet very little luminosity; the ratio now becomes 50-1. This is another piece of evidence for dark matter.
There are two proposed dark matter categories. These are the MACHOs and the WIMPs. MACHOs are massive compact halo objects. In other words, this dark matter is what makes up humans, planets, brown dwarfs and more. Since MACHOs are not very luminous, scientists use the gravitational lensing effect to detect them. Whenever collections of MACHOs pass in front of a star, they curve much of the star's light, creating either a stronger signal, or multiple copies of the same star; it is a type of optical illusion for us. The other hypothesized dark matter, WIMPs (weakly interacting massive particles) is said to make up the bulk of the universe's mass. These particles would resemble neutrinos, but would be much slower, more massive and would evade any direct detection. These WIMPs would not emit electromagnetic energy and therefore would not be able to "clump" into concentrated agglomerations such as the bright galaxies we see today. In other words, the WIMPs would remain in orbit around the center of the galaxy and would not interact with anything to any great degree.