MIT astrophysicist Peter Fisher explained the bullet cluster (background) to an audience in Brooklyn. In the image behind him, two galaxies (red) collide with each other but the dark matter (blue) is unaffected and pulls ahead, separating from the galaxy cluster. [Image credit: Dan Robitzski]
There’s a lot of stuff in the universe. Billions and billions of galaxies, each containing billions and billions of stars, all sail away from each other, carried by the universe’s constant expansion. And yet, all this visible stuff we’ve found counts for just a small percentage of what’s out there. Luminous matter — everything that we interact with and see — makes up only about 15 percent of all the mass in the universe. Dark matter, named so for its complete invisibility, accounts for all the rest. But for all its abundance, scientists have no idea what it is.
I went back to Pioneer Works, the Brooklyn museum and science outreach center, for the latest in their series of “Science Controversies” panels. Sitting all around me in the large, open-floor building were fellow physics enthusiasts, most of whom had traded in a suggested donation for a plastic cup of wine. Once more, three decorated physicists tried to answer the big questions: What is this stuff? What is the dark matter that makes up the majority of our universe and will we ever be able to “see” it for ourselves? Much like my last visit, there were few definitive answers but lots of good guesses.
“Everything we have ever seen, which has been brought to us by light predominately, all of that makes up a tiny fraction of what’s out there,” said host Janna Levin, the science director at Pioneer Works and an astronomer at New York City’s Barnard College. We can see luminous matter because when electromagnetic radiation — in the form of a beam of light — hits something, it bounces off and reflects an image that our eyes can pick up.
When it comes to dark matter, however, the light passes right through. No reflection, no appearance at all. “Everything we know about the things we interact with are based on their electromagnetic interactions,” added panelist Elena Aprile, an astrophysicist at Columbia University.
We may never have directly observed dark matter, but physicists have strong evidence indicating that it’s out there. When they looked into how galaxies formed during the early moments of the universe, their findings were problematic. With just luminous matter, galaxies simply didn’t have enough mass to form or maintain a structure. “In order for them to have formed, there must be dark matter,” said Levin, explaining how galaxies required the gravitational forces provided by the extra mass, not unlike an invisible foundation, in order to stay together. “Whatever dark matter is, it’s crucial to the formation of all the structures in the universe.”
If there were no dark matter, said panelist Peter H. Fisher, a particle physicist at the Massachusetts Institute of Technology, “[galaxies] would basically fly apart.” He explained that scientists realized something was missing when the critical density of the universe — the amount of matter necessary to keep the universe from expanding uncontrollably or collapsing — was calculated to be ten times the amount of visible matter. “The best way to think of a galaxy is a ball of dark matter with some regular matter spaced throughout,” Fisher said.
We don’t see that big ball of dark matter, but we do see what it does. A fundamental concept of physics is that gravity pulls matter together. Gravitational forces guide planets and stars along their orbits and can even bend light. As light travels from the luminous matter of distant galaxies to us, the dark matter it passes along the way bends and distorts the image.
“All the evidence [for dark matter] is found through the gravitational effects that dark matter has on everything we see and observe,” said Aprile.
If the universe were the empty void it appears to be, the light bouncing off these distant objects would travel in a straight line. But because the images appear in the form of distorted arcs and circles — and because bent light can travel around objects that would normally be in the way — we know that there is quite a bit of mass out there. By reconstructing the image through a complicated process called gravitational lensing, scientists like Aprile can calculate precisely how much mass, in the form of dark matter, the light passed. This process essentially reverse-engineers the universe based on the extent to which light is affected.
But for everything scientists do know about dark matter, there is still no consensus on what dark matter is made of. “This material is fundamentally different from anything [else] in the universe,” said Levin.
The leading explanation for dark matter comes from recent research into the nature of WIMPs, or weakly-interacting massive particles. Ultra-lightweight — the word massive only finding its way into the acronym because WIMPs do in fact have mass — WIMPs are a proposed class of particles that don’t interact with other particles except through gravity, which is pitifully-weak at small scales. “A billion particles of dark matter could be passing through us every second and we would never know it,” said Levin.
When researchers calculated how many WIMPs, which would have been generated during the big bang, ought to remain in the universe, the number matched the quantity of actual dark matter calculated to be out there. At the Large Hadron Collider in Geneva, experiments designed to recreate the conditions of the early universe are currently underway. “The real hope now is to create dark matter,” said Levin, explaining that the experiments are trying to generate neutrinos — tiny particles that we already know exist — to test whether or not they are a good candidate for dark matter. Neutrinos have mass but rarely interact with other particles, so some physicists think they may be the WIMPs responsible for dark matter.
Aprile also thinks that dark matter is likely made of WIMPs, but her experiments take a different approach than those at the collider. After years of planning and construction, her Xenon1T study has begun collecting data on neutrinos from sensitive detectors shielded deep underground. The hope is that by setting up the most sensitive apparatus possible, any neutrino passing by could be detected.
“I always have to explain that I don’t see when the WIMP interacts,” said Aprile. “We have to do lots of analysis with millions and millions of data.”
So far, dark matter remains hidden. That may be because the technology isn’t there yet, or because dark matter isn’t made of neutrinos — which are just one of many types of WIMPs — after all. “We all agree that this may be the wrong way to detect dark matter,” Aprile said, “but if we don’t do it then we won’t know.”
Fisher suspects that the latter might be the case. After his own experiments — like all others before it — failed to detect WIMPs, Fisher moved on to a new search. Now he suspects that the key to understanding dark matter might be a MACHO: Massive Compact Halo Object.
Arguably the most MACHO objects in the universe, black holes could provide the galaxy-stabilizing mass that researchers are looking for. Fisher explained an idea originally proposed, then written off, and then once more supported by Stephen Hawking: that during the big bang, it was possible to get enough energy together in the same place to form small, primordial black holes.
These black holes could be very small — even as small as a proton — but because black holes are so dense, a black hole that size would have as much mass as the moon. MACHOs spread throughout the universe, some in the centers of galaxies and stars, would give off enough gravitational forces to fit the bill as a candidate for dark matter.
MACHOs are no easier to spot that WIMPs, unfortunately. “One of these could go zipping through the earth and we wouldn’t really know it. It might scoop up a few kilograms,” said Fisher.
Both Fisher and Aprile conceded that they didn’t really know who was right. “[Dark matter] may be particles, it may be primordial black holes, something else. It may be anything,” said Aprile. “As curious human beings, we just have to keep searching and find out what this damn thing is.”