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Mysterious Dark Matter Blob Confounds Experts
mayberry42 writes "Astronomers using the Hubble Space Telescope are mystified by a merging galaxy cluster known as Abell 520 in which concentrations of visible matter and dark matter have apparently come unglued. A report on the Hubble observations, published in the Astrophysical Journal, raises more questions than answers about a cosmic pile-up that's occurring 2.4 billion light-years away. 'According to our current theory,' says Arif Babul, the study team's senior theorist, 'galaxies and dark matter are expected to stay together, even through a collision. But that's not what's happening in Abell 520. Here, the dark matter appears to have pooled to form the dark core, but most of the associated galaxies seem to have moved on.'"
One way to figure out gravitational field from a static image is to look at galaxy distribution behind the gravitational field. If it is squishes space in one direction while stretching it in the other, you will see more galaxies longer in one direction then in the other, so you can build a map of distortion and compute gravitational field from it. The result will be coarse, but you will see large concentrations of matter.
They detected it by gravitational lensing. http://en.wikipedia.org/wiki/Gravitational_lens. The dark matter is massive enough that it bends the light passing through it. So you can for example see that a star looks bent and not as spherical if it is behind a lot of dark matter. In the really blatant examples of gravitational lensing you get things like the Einstein Cross http://en.wikipedia.org/wiki/Einstein_Cross where you can see multiple copies of the same object.
Here is general description and large annotated Hubble image
There's pedantry which serves the useful purpose of correcting other people's mistakes, and then there's pedantry of the "look how clever I am" variety; posts like yours, which seem to get posted to every single story on any kind of astronomical event that takes place outside the solar system, are examples of the latter.
There is an excellent word for this and it means far more than just "pedant" and it's Finnish.
The word is pilkunnussija, literally "comma fucker"
The more you know.
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BMO - perkele
http://www.youtube.com/watch?v=7ImvlS8PLIo#t=1582s Worth watching the whole thing, but this portion briefly addresses gravitational lensing.