The world we see and know is made up of normal matter. Anything made of atoms is considered normal matter. Normal matter makes up five percent of all matter. So what constitutes the rest of the ninety-five percent? Dark energy makes up about seventy percent of our universe, while dark matter makes up the last twenty-five percent. What is dark matter? Dark matter is the only explanation we have for why our universe expands the way it does, but we have no idea what it's made of. Dark matter doesn't clump together like normal matter, so it doesn't make up anything we find important, like planets and life forms. Instead, dark matter extends across our galaxy, spanning millions of light years. The spread of dark matter is much larger than that of normal matter, for example our Milky Way is only about a few thousand light years thick and about 100,000 light years in diameter , but that's exactly what we see, normal matter. Dark matter extends even further than normal matter over millions of years in all directions. These two types of matter affect gravitation, but to what extent? The mass of normal matter, like that of objects like the sun, is what makes up most of the gravitational attraction. It has been calculated that dark matter has half the effect that the sun has on Earth. To truly see dark matter in action, you need to step back and look at our entire galaxy. Dark matter works in a much broader aspect. It's easier to see the effect of dark matter from farther away because of how it extends further than normal matter. Given the gravitational pull of normal matter, stars closer to the center of a galaxy should move at a faster speed than stars farther from the center. However, in the 1970s, Vera Rubin discovered that all stars...... middle of paper .......d.). Retrieved from http://science.howstuffworks.com/dictionary/astronomy-terms/dark-matter.htmChase, S. (nd). Retrieved from http://math.ucr.edu/home/baez/physics/Relativity/GR/dark_matter.html http://www.washingtonpost.com/blogs/innovations/wp/2013/11/07/dark-matter- what we know and why you should care/Al-Khalili, J. (November 18, 2013). How close are we to finding dark matter?. Retrieved from http://www.bbc.co.uk/news/science-environment-24987749http://www.seasky.org/celestial-objects/dark-matter.htmlRubin, V. (nd). Retrieved from http://www.indiana.edu/~geol105/images/gaia_chapter_1/dark_matter_in_the_universe.htmhttp://www.learner.org/courses/physics/unit/text.html?unit=10&secNum=2Charles, C. ( nd). Retrieved from http://www.huffingtonpost.com/2013/12/08/dark-matter-black-holes_n_4408646.html http://space.about.com/od/deepspace/a/What-Is-Dark-Matter. htm