Expert: Jayendra Upadhye - 8/21/2008

Question
what is dark matter.and information about dark matter.

Answer
Hi Amjad,
Dark matter was postulated for quite some time, but it first came to be seriously considered when people began studying the velocity curves (plotted against radial distance from center, of the stars in spiral galaxies.

What the found was that In the hub region, the velocity increased linearly with increasing distance from the center, which was as per expectation.

But at the boundary of the hub with the galactic disk, the curve flattened out! measning the velocity of the stars did not show appreciable change over radius from center!

This led scientists to postulate that there was 10 times more hidden mass in the galactic discs of spiral galaxies than was "visible".

On the other hand the Lambda-CDM theory also postulates dark matter playing a role in the early history of the universe!

Other evidence was the gravitational lensing associated with huge clusters of galaxies such as the Virgo cluster, Coma cluster.
(Abell cluster being most famous for its gravitational lens displaying arcs of light with red shift 7!! making it the most distant light seen by humans yet! (check me out about the 7 figure, i am quoting from memory).

Read all about dark matter at
link1 - http://en.wikipedia.org/wiki/Dark_matter
link2 - http://en.wikipedia.org/wiki/Lambda-CDM_model
link3 - rotation curve flattening:- http://en.wikipedia.org/wiki/Image:GalacticRotation2.svg
link 4- dark matter overlay:-
http://en.wikipedia.org/wiki/Image:060821_darkmatter.jpg

hope that suffices.

Please do rate the answer if you found it helpful.
regards
Jayen