StarChild Question of the Month for July 2003

Question:

How do we know that dark matter exists?

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Answer:

Dark matter is the name scientists have given to the particles which we believe exist in the universe, but which we cannot directly see! Dark matter was initially called "missing matter" because astronomers could not find it by observing the universe in any part of the electromagnetic spectrum. This material appears to have mass (and therefore generates gravity), but it does not appear to absorb or emit any electromagnetic radiation. Given the fact that it does not send us any light (which is how we have learned most of what we know about the universe), it is not difficult to understand that it has been hard to discover anything about the nature of these mysterious particles. But do not despair, scientists are coming up with clever new ways to probe the dark parts of the cosmos!

Scientists study dark matter by looking at the effects it has on visible objects. Scientists believe that dark matter may account for the unexplained motions of stars within galaxies. Computers play an important role in the search for dark matter information. They allow scientists to create models which predict galaxy behavior. Satellites are also being used to gather dark matterinformation. In 1997, a Hubble Space Telescope image revealed light from a distant galaxy cluster being bent by another cluster in the foreground of the image. Based on the way the light was bent, scientists estimated the mass of the foreground cluster to be 250 times greater than the visible matter in the cluster. Scientists believe that dark matter in the cluster accounts for the unexplained mass.

More recently, astronomers have used NASA's Chandra X-ray Observatory to make the most detaile measurement yet of the distribution of dark matter in a massive cluster of galaxies. Chandra observed a cluster of galaxies called Abell 2029 located about a billion light-years from Earth. The cluster is composed of thousands of galaxies enveloped in a gigantic cloud of hot gas, and an amount of dark matter equivalent to more than a hundred trillion Suns! The hot gas in the cluster is held in the cluster primarily by the gravity of the dark matter, so the distribution of the hot gas is determined by that of the dark matter. By precisely measuring the distribution of X-rays from the hot gas, the astronomers were able to make the best measurement yet of the distribution of dark matter near the center of a galaxy cluster. Extrapolating from their results, their results indicate that about 80 percent of the total amount of matter in the universe consists of a form of dark matter called "cold dark matter" - to distinguish it from "hot dark matter". Cold? Hot? These terms do to refer to the temperature of the particles, but to the speed at which the dark matter particles were moving when the galaxies began to form billions of years ago. According to computer models, there are two possibilities for how dark matter would have been behaving at that time depending on exactly how and when it formed. Cold dark matter would have been moving slowly by the time of galaxy formation; hot dark matter would still have been moving quickly.

Composite image showing large cloud of hot gas as determined by the Chandra Observatory X-ray observation and the 
smaller visible galaxies comprising the cluster which are embedded the the cloud (as imaged by the Digital Sky Survey)
This composite image (Chandra X-ray image on the left and DSS optical image on the right) of the galaxy cluster Abell 2029 shows a huge hot gas cloud (as seen in X-rays) envelopes the galaxies in the cluster (the bright spots in the visible image). The cluster does not behave as scientists would expect it to if only the visible matter is generating the gravity present in the cluster. 'Dark matter' theory suggests that a huge amount of dark (invisible to direct observation) matter, interacting gravitationally with the normal, visible matter in the universe, exists.

So while we still do not know the exact nature of these particles, we have at least learned that they were moving slowly at the time galaxies formed in the early universe. This is new data for the theorists to use in continuing to develop their models.

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