This composite image shows the galaxy cluster 1E 0657-56, also known as the “bullet cluster.” This cluster was formed after the collision of two large clusters of galaxies, the most energetic event known in the universe since the Big Bang.  Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.

Matter, we like to think, is something we can see, something we can touch. But the substance that cosmologists call dark matter and insist constitutes more than 20 percent of our universe—more than five times the quantity of ordinary matter—has neither of these properties. However, an ever-growing accumulation of gravitational anomalies in our universe makes denying dark matter’s existence nearly impossible.

Scientists offered powerful confirmation of dark matter this week, after observing a collision of galaxies in the so-called bullet cluster.

Post-impact, the bulk of the cluster’s mass lay outside the gas that is actually observable, meaning ordinary matter cannot account for all of the mass, the researcher said. Therefore, dark matter is necessary to explain the data. That result, which will be published in forthcoming issues of Astrophysical Journal and Astrophysical Journal Letters, may provide the most compelling evidence of dark matter to date.

“I don’t think there is any other way to work around it,” said Marusa Bradac, one of the study’s authors and a researcher at Stanford’s Kavli Institute for Particle Astrophysics and Cosmology. “This is the clear-cut example where we do see there is something else in the system.”

The bullet cluster formed when two different groupings of galaxies passed through each other. As the two galaxy clusters collided, the visible gas—which constitutes regular matter—slowed down as a result of frictional forces. But dark matter, which gives off no light or heat, is not subject to these same forces and would not have slowed down during the collision. The researchers hypothesized that if the colliding galaxy clusters contained dark matter, the visible and invisible matter would have separated as the dark matter traveled more rapidly past the collision site. 

When the scientists measured the mass after the collision, they found that much of the bullet cluster’s mass lay beyond the observable gas, where they had predicted the dark matter would be.

“Because normal and dark matter usually are closely intertwined in galaxies and clusters, it has always been difficult to distinguish unambiguously between the dark matter and the modified gravity paradigms,” Maxim Markevitch, a Harvard University astronomer and another of the study’s coauthors, wrote in an email. “This is the first time when dark matter and normal matter have been seen separated in space, and the reason is the energetic cluster collision.”

Given Newton’s laws and the amount of visible matter in the universe, galaxies do not rotate as expected. Though the idea of dark matter has become a popular way to resolve this problem, some scientists favor modifying Newton’s laws instead.

Mordehai Milgrom, the man who originally proposed Modified Newtonian dynamics (MOND) as an alternative to dark matter, said that just because the researchers didn’t see all the matter they measured doesn’t mean dark matter had to be present.

“Everyone knows there is still a lot of normal matter out there that we have yet to detect,” he said via email. “While this is also ‘dark,’  it is not what we call the ‘dark matter,’ which people say has to be there. So it is definitely not a proof of the dark matter paradigm.”

The new study’s authors say their paper is not an attempt to disprove MOND, but insist that their results point directly to dark matter.

Other cosmologists agree with their assessment.

“It will always be possible that some sort of modification of gravity lurks just below our threshold of detection,” Sean Carroll, a University of Chicago cosmologist, wrote on his blog, Cosmic Variance. “But now we have established beyond reasonable doubt that we need a substantial amount of dark matter to explain cosmological dynamics.”

Originally published August 25, 2006

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