Dark matter 'ghosts' through galactic smash-ups
By Jonathan Webb
Science reporter, BBC News
By observing multiple collisions between huge clusters of galaxies, scientists have witnessed dark matter coasting straight through the turmoil.
Dark matter is the mysterious, invisible stuff that makes up 85% of the matter in the cosmos - and these results rule out several theoretical models put forward to explain it.
This is because it barely interacts with anything at all, including the dark matter in the oncoming galaxies.
The work appears in Science magazine.
To conduct their study, astrophysicists looked at 72 smash-ups between galactic clusters, using two space telescopes: visible light was recorded by the Hubble Space Telescope, and X-rays by the Chandra Observatory.
Scouring multiple views of the collisions, the researchers tracked the movement of the three main components of galaxies: stars, clouds of gas, and dark matter.
The violently swirling clouds of gas are hot enough to glow with X-rays, which Chandra detects. And stars can be seen in regular, visible-light images from Hubble.
Dark matter is more difficult to "see" - but not impossible. Although it does not emit or absorb light, it does have gravity, and so it bends the path of light passing nearby. This warps our view of anything on the other side of it, in an effect called "gravitational lensing".
"Looking through dark matter is like looking through a bathroom window," said Dr Richard Massey from Durham University, one of the study's authors. "All the objects that you can see in the distance appear slightly distorted and warped."
Using this distortion allowed Dr Massey, with colleagues from the University of Edinburgh, University College London and Switzerland's Ecole Polytechnique Federale de Lausanne (EPFL), to "map" the dark matter in the clusters as they collided.
Galaxy clusters are vast and contain huge amounts of dark matter, so when they collide - over billions of years - it offers a unique glimpse of how the stuff behaves.
"We like these collisions because it's exactly what we'd do in the lab," Dr Massey told BBC News.
"If you want to figure out what something is made out of, you knock it, or you throw it across the room and see where the bits go."
In this case, the bits went straight through each other.
Unlike the gas clouds, which grind to a turbulent halt, and the stars, which mostly glide past each other, the ubiquitous dark matter passes through everything and emerges unscathed, like a ghost.
"It seems not to interact with anything at all," Dr Massey said.
Earlier observations of the "Bullet Cluster" - a bust-up between two particularly big groups of galaxies, now in its final stages - had already demonstrated dark matter's weird lack of interactions, including with itself.
But this new, major survey was able to deliver much more precision, concluding that there was even less interaction than the previous work allowed for.
"If you bang your head against the wall, the electrostatic force between the molecules in your head and the ones in the wall cause a collision. This is what dark matter doesn't seem to feel," Dr Massey explained.
Dark matter does "feel" gravity; those interactions are the reason we know it is there, and the reason it is bound up in the galactic collisions to begin with. But the lack of almost any other interaction makes it even more mysterious than before.
Link to the original BBC article:
http://www.bbc.com/news/science-environment-32066013
The original article in Science Magazine:
http://www.sciencemag.org/content/347/6229/1462
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