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Astronomers from Johns Hopkins University and the Space Telescope Science Institute in Baltimore, Md. have come up with the first detailed map of the mysterious dark matter, albeit one covering only a tiny patch of the sky.
Myungkook James Jee, Johns Hopkins researcher and co-author of the project, said that the team came up with the map by measuring the effects of gravitational lensing by dark matter on the images of two galaxy clusters taken by the Hubble Space Telescope.
Gravitational lensing is a visual effect postulated by Einstein's general theory of relativity. Any sufficiently massive body will cause a dimple in space-time and shift, distort, even multiply images of stars and galaxies hiding behind it like a fun-house mirror.
"The images we took show clearly that the cluster galaxies are located at the densest regions of the dark matter haloes, which are rendered in purple in our images," Jee said.
The astronomers also reported that dark matter has a ghost-like character where dark matter particles can pass through each other rather than bounce off and scatter like billiard balls as is common with ordinary matter. "Collision-less particles do not bombard one another, the way two hydrogen atoms do," said Jee. "If dark matter particles were collisional, we would observe a much smoother distribution of dark matter, without any small-scale clumpy structures."
The map appears in the December edition of the Astrophysical Journal.

Image: Snapshot of the computer simulation of the dark matter Universe. These filamentary structures are called "cosmic webs" of dark matter. Credit: Johns Hopkins University.