We know dark matter is out there—but what is it?
An invisible army of black holes? A cosmic graveyard of burned-out stars? A swarm of rogue planets that roam the depths of interstellar space? While examples of objects like these have been observed, we now know that they can’t account for the enormous mass of dark matter required to explain why galaxies rotate so fast. Following Sherlock Holmes’ dictum that once you have ruled out the impossible, whatever remains, however improbable, is the answer, scientists have been forced to conclude that dark matter is an entirely new form of matter, never before observed.
Here is what we think: Every galaxy is engulfed by a cloud of dark matter particles that extends far beyond that galaxy’s visible edge. Each dark matter particle is electrically neutral and has a mass tens or thousands of times that of the familiar proton. Finally, there is a lot of this dark matter. Our best estimate is that there is about five times as much dark matter as there is luminous matter, making our visible universe a thin frosting on a dark matter cake.
But physicists will need to observe dark matter first-hand before anyone should believe it is real. Our search for dark matter takes three distinct approaches: direct, indirect, and production—that is, actually making our own dark matter particles.
The direct approach starts with a detector cooled to more than 459 degrees below zero Fahrenehit, so close to absolute zero that the atoms that make up the detector are nearly stationary. The detector is buried as much as a mile underground to protect it from ordinary cosmic rays, high-energy particles that are constantly bombarding the Earth. Though these detectors can’t actually “capture” a dark matter particle, should one happen to pass through and collide with the nucleus of an atom inside the detector, the detector will ring like a bell and the passage of the dark matter particle will be observed.
There are dozens of experiments underway using this approach, including one, called the DAMA (DArk MAtter) experiment, that has made a provocative finding. Scientists think that dark matter flows past the solar system like a wind, so DAMA uses the motion of the Earth around the Sun to winnow out a dark matter signal. For half a year, the Earth is moving into the dark matter wind, and for the other half, it is moving with the wind. Therefore, we expect to see an annual variation in the number of dark matter hits. This is exactly what DAMA has seen for many years now.
The problem is that other experiments which are nominally more sensitive don’t see this annual variation. This has led to considerable confusion and it will take additional work to understand if DAMA has seen the first hints of dark matter or merely an unexplained measurement artifact.
Indirect searches exploit the notion that dark matter might consist of both a matter and antimatter component. If so, occasionally a pair of matter and antimatter dark matter particles might meet and annihilate each other in a flash of gamma rays or matter/antimatter pairs that can be observed by satellites that are designed to detect gamma rays or antimatter in the cosmos. In fact, two such experiments, PAMELA and GLAST, have observed signals that could be the signature of dark matter, but could also have more prosaic explanations. Meanwhile, other experiments see no such signals.
Rather than waiting for dark matter to come to us, though, some physicists are hoping to make their own dark matter right here on Earth. Currently the only particle accelerator capable of making dark matter is the Large Hadron Collider at CERN. By exploiting Einstein’s famous equation E = mc2, we hope to convert the kinetic energy of the beams directly into dark matter. Because dark matter is electrically neutral, it would escape our detectors undetected, but upon adding up the energy contained in all the particles that we can detect, we would notice that the energy books are unbalanced and that some energy is missing.
The scientists working on two of the LHC’s detectors, the ATLAS (A Toroidal Large Apparatus) experiment and my own CMS (Compact Muon Solenoid), are now searching their data tirelessly, looking for collisions with these characteristics. The situation is evolving rapidly as the LHC delivers a torrent of particles to the detectors.
It’s a race between the three different approaches to see which one will be the first to observe a reliable signature of dark matter. No one should be the slightest bit convinced until at least two of the approaches begin to tell a consistent story. One thing is certain; with five times as much dark matter as ordinary matter, the race is on for discovery and Nobel Prize glory.
Editor’s picks for further reading
CERN Courier: Shedding Light on Dark Matter
In this article, learn how scientists are using the CMS instrument at the LHC to look for signs of dark matter.
NOVA scienceNOW: Dark Matter Mystery
In this video, host Neil deGrasse Tyson reports from a half mile underground in an abandoned mine, where scientists are using special detectors to look for dark matter particles.
Scientific American: Instant Egghead: Dark Matter
Scientific American editor George Musser explains dark matter in 60 seconds using a coffee cup, some crumbs, and a compact disk.