What is Dark Matter? — Mark from Accuweather.com

Apr 6, 2011; 9:06 PM ET
A fan of AccuWeather Astronomy on facebook mentioned a TV show they watched about dark matter. This subject is very interesting and since I did not know much about it, I did some research on what it is and decided to share it with you.
Dark matter is the name given to the amount of mass whose existence is deduced from the analysis of galaxy rotation curves which they had no explanation for. Even though we do not see dark matter directly, its gravitational influence can be seen in the motion of gas and stars in galaxies, and also in the motion of hot gas and galaxies within clusters of galaxies.
There are many theories on what dark matter could be. At the moment there is not a convincing enough answer and the question is still a mystery.
Astronomers know that the mass needed to cause the motions is out there. This mass in turn leads to the gravity needed to cause these motions. Remarkably, it turns out there must be at least five times more mass in clusters of galaxies than we see. Since we can not see it, we assume it is dark and thus the term dark matter was coined by a Swiss astronomer named Fritz Zwicky in the 1930s.
In turns out that most of the stuff in clusters of galaxies must be invisible due to the amount of gravity needed to cause the previously unexplained motions. Astronomers then conclude that in fact most of the matter in the entire universe is invisible.
Just imagine then that what we see out there is just the tip of the iceberg. There must be so much out there that we do not even know is there.
An Overlay of an Optical Image of a Cluster of Galaxies
with an X-Ray Image of Hot Gas Lying Within the Cluster

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