Dark Matter in the Universe

One of the biggest discoveries of the 20th century in astronomy was the fact that dark matter existed. It was found out that the baryonic matter--also known as the electrons, protons, and neutrons that make up the visible universe--made up only 5% of the universe. It was found that dark matter made up another 25% of the known universe. Dark matter is, as of right now, completely invisible to all types of detection. The only reason we know that it exists is because of the effect it has on the visible portion of the universe.

(The image, created by NASA, gives a much more exact percentage signed to all of the matter in the universe.)

For example, in the 1930s, scientists measured groups of galaxies, and determined that there wasn’t enough visible stuff to exert enough gravity to hold them together. This was later discovered to be the same for individual galaxies. Thus, something has to be holding the galaxies together. They called this something  dark energy.

Another example is that stars on the edges of a spinning spiral galaxy, according to standard physics, should be spinning more slowly than the stars in the center of the galaxy. However, it was found that they all spin at pretty much the same speed, which gives credence to the fact that dark matter exists, because something has to be exerting a force on the outer stars for them to be traveling at the same relative speed.

(This photo shows a spiral galaxy in the center, with the luminous halo around it with globular clusters throughout, and the dark matter halo surrounding it.)
Sources:

http://science.nationalgeographic.com/science/space/dark-matter/

https://www.youtube.com/watch?v=HneiEA1B8ks

http://scienceblogs.com/startswithabang/files/2012/07/I02-13-composition21.jpeg

darkmatter.jpeg