Dark Matter

Dark matter makes up more than 90% of the universe, yet because it neither emits nor reflects electromagnetic radiation, it cannot be observed directly.

Yet a group of astronomers in Baltimore have used the Hubble Space Telescope "to map the dark matter billowing out from the long-ago collision of two galaxy clusters."

The photo released by NASA is quite extraordinary:


The Baltimore Sun explains: 

They’re calling it the strongest evidence yet of the existence of dark matter, and the first observation to separate it from its associated stars, galaxies and glowing gas.

"What we found is a very peculiar structure – a ring-like structure that surrounds the core of the cluster," said Johns Hopkins University research scientist M. James Jee, lead author on the study that will appear in the June 1 issue of The Astrophysical Journal.

It’s not exactly a ring of dark matter, he said. Rather, it’s a map of where the densest regions of dark matter must be, based on measurements of how that mass and gravity are bending the light streaming by from galaxies far in the background….

Jee likened the ring (in three dimensions actually a flattened spherical shell) to a jam-up of dark matter particles hurled outward from the collision, like commuters headed out of town, backing up behind slowing traffic ahead.

The Sun article notes that there is skepticism among some astronomers, who would like to see the evidence captured from a second source to rule out "peculiarities" in Hubble’s camera. Perhaps the image above is simply the astronomical equivalent of refracted light on film appearing as a "ghost" in your family photographs. Nonetheless, the evidence for dark matter is there, whether we’ve observed it yet or not.

(Also be sure to click here for a related video simulation of two galaxy clusters colliding.) 

Watch this Hubblecast to better understand the significance of this finding.