Photograph by ESO
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This image from the NASA/ESA Hubble Space Telescope shows four merging galaxies. The dark matter “halo” surrounding one of them seems to be off-center, suggesting that dark-matter particles are feeling some unseen, unknown force.

Photograph by ESO

Dark Matter May Be Less Mysterious Than We Thought

Unlike ordinary particles, dark matter seems to feel only gravity, not the other forces of nature. But new observations show that might not be true after all

Astronomers have been struggling for nearly 80 years to figure out what makes up the mysterious dark matter that pervades the universe. They came up with the idea of it to explain why, judging from the motions of galaxies, the universe seems to have so much more mass than the visible stars and galaxies can account for.  They strongly suspect it’s some sort of exotic subatomic particle, created in the Big Bang in mind-blowing quantities and not visible by any means except by the pull exerted by its gravity. The particle notion is still mostly theoretical, given that astronomers have not yet detected one of these particles directly. (See A First Glimpse of the Hidden Cosmos).

But a new set of observations by the Hubble Space Telescope and by the Very Large Telescope, based in Chile, may have just come up with a crucial clue about what the dark matter actually is. (See New Eyes on the Universe). A new paper appearing in Monthly Notices of the Royal Astronomical Society argues that dark matter particles within a galaxy cluster known as Abell 3827, about 1.4 billion light-years from Earth, are responding to a force other than gravity.

What the paper’s authors actually detected was an unusual arrangement of dark matter. Normally, dark matter forms a huge halo around a galaxy. Our own Milky Way’s halo is part of what keeps us together: The galaxy is spinning so fast it would fly apart without the extra gravity of dark matter to hold it together. But for one of the galaxies in this cluster, the center point of the dark-matter halo is offset by about 5,000 light-years the center point of the galaxy itself.  

In space, that’s not very far. But it should be zero if dark-matter particles are subject to gravity alone, as some theories suggest.

But other theories predict that another force should be involved. For at least a couple of decades now, theorists have leaned toward something called “weakly interacting massive particles, (WIMPS) as the best explanation for what dark matter is made of. Such particles would feel not just gravity, but another, relatively weak force—something similar to the so-called weak nuclear force, which among other things can trigger radioactive decay.  

This could be the first experimental evidence that dark matter is indeed made of WIMPS. The best explanation for the dark-matter cloud’s offset, says Massey, is that it’s passing through other dark-matter clouds in the core of Abell 3827. Friction between the clouds, caused by this extra force, is forcing the one Massey observed to lag behind its galaxy—although Massey and his colleagues have no idea yet what the force is. An extra force besides gravity wouldn’t make dark matter less dark in the sense of being easier to see directly, but rather in the sense of casting some light on its nature.

They’re also open to the possibility that something much more mundane is going on. A burst of star formation on one side of the visible galaxy, for example, could create a bright spot that could skew the astronomers’ estimate of where the galaxy’s center is. Or perhaps the gravity of nearby galaxies, might be distorting the visible galaxy’s shape, again making its center hard to pinpoint. “It’s tough to think of a convincing alternative explanation,” Massey says, “but this is such an exciting discovery that I’m being extra super cautious rather than shouting from the rooftops.”

The idea that dark-matter particles are slowing each other down through some unknown force “does seem like the most likely explanation at this point,” says Jason Rhodes, a dark-matter expert at NASA’s Jet Propulsion Laboratory. “But we clearly need more evidence.” The search for that evidence is already under way, with a set of new observations of other galaxies planned.

Even if more examples of offset dark matter are eventually found, it won’t necessarily nail down the nature of the particles in question. Even if they do feel some force in addition to gravity, it will be tricky to calculate the strength of that force, and thus to narrow down the candidate list of possible particles even further.

Ideally, particle physicists at Europe’s Large Hadron Collider, the world’s biggest particle accelerator, will manufacture some dark-matter particles here on Earth, where their properties will be easier to measure. Or maybe one of several underground detectors, designed to snag a particle as it passes through the Earth, will ping with a telltale electronic signal.

One way or another, physicists and their astronomical brethren are convinced that the mystery of dark matter can’t remain dark forever.

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