The cosmic collision of two galaxy clusters has given astronomers a clearer look at the mysterious substance known as dark matter. Researchers say when the two clusters crashed into each other, the
dark matter from each cluster [appeared] to pass through the cosmic mess unscathed, leaving ordinary matter behind in the galactic pileup [SPACE.com].
Dark matter, mysterious stuff that exerts a gravitational force on other matter, was originally proposed to explain what holds spinning galaxies, like the Milky Way, together. Observations suggest it outweighs ordinary matter by a factor of about 6 to 1. But no one knows what it is made of, and normally dark matter and ordinary matter are too well mixed to observe the dark matter independently [New Scientist].
The new study, which will be published in a future issue of the Astrophysical Journal [subscription required], suggests that the galaxy clusters collectively known as MACS J0025 were moving at a speed of millions of miles per hour when they collided. In the melee,
hot gas from each cluster collided and slowed down, but the dark matter did not. That separation provides evidence to support the view that dark-matter particles interact with each other only very weakly or not at all, apart from the pull of gravity [MSNBC].
While researchers can use the new data to study how dark matter interacts, it won't give them any insight into the substance's composition, which looms as one of the biggest unanswered questions in astrophysics. Some scientists have hypothesized that dark matter is composed of exotic subatomic particles that they call weakly interacting massive particles (WIMPs), but these oddities have never yet been detected. Image: X-ray(NASA/CXC/Stanford/S.Allen); Optical/Lensing(NASA/STScI/UC Santa Barbara/M.Bradac)