In this case, dark is no misnomer.
So little is known about dark matter — and its unrelated but similarly enigmatic cousin, dark energy — that scientists aren’t even sure it really exists. But two separate teams of physicists researching the elusive stuff have now come to very similar conclusions about its composition, based on data taken by an instrument on the International Space Station.
One of the leading theories for exactly what dark matter could be has been around for decades, and suggests that it is comprised of Weakly Interacting Massive Particles, or WIMPs. The “weakly interacting” part comes from their assumed interaction with other matter via the weak force (and gravity), one of the four known fundamental forces in the Universe, and “massive” because they are much larger than other standard particles — roughly 85 times the mass of a proton, for example.
Analysis of how galaxies rotate in the Universe suggests that there could be a great deal of dark matter out there, exerting its gravitational pull on the matter that we can see. But therein lies the problem: dark matter is invisible.
Having no known interactions with the electromagnetic force, our usual astronomical techniques, which rely on picking up light from distant sources, are rendered useless. Direct detection of dark matter has therefore proven tricky, but scientists have another trick up their sleeve, as has been evidenced in these new results.
Particle theory tells us that these theoretical WIMPs should naturally annihilate with each other. By doing so, their mass is converted into other products, including gamma rays, neutrinos and other exotic material, such as antimatter. With special detectors we can detect this assortment of cosmic rays, with one such instrument on the International Space Station (ISS) doing just this, the Alpha-Magnetic Spectrometer (AMS-02)
The AMS-02 recently released a trove of data containing detections of antiprotons, the antimatter counterpart of normal protons. With this very precise data, two separate teams were able to eke out the number of antiprotons that were likely coming from other known sources in the Universe, such as exploding stars — supernovae — and the turbulent cores of distant galaxies known as active galactic nuclei (AGN). The remaining detections were therefore unaccounted for.
The theory is that these antiparticles with no defined origin could be the result of dark matter annihilations, suggesting that WIMPs could be the much-desired explanation for one of the greatest mysteries in modern physics.
One team, led by Alessandro Cuoco of RWTH Aachen University in Germany, focused on computer simulations to show that a Universe with dark matter best accounts for the antiprotons of unknown origins, and that such WIMPs would likely have a mass of about 80 GeV, which fits with theoretical models.
Another team, led by Ming-Yang Cui of the Chinese Academy of Sciences, took an entirely separate approach and looked more towards chemistry. As a cosmic ray containing the element boron travels through space on its way to us, some of it decays into carbon. By measuring the amount of decay, scientists can calculate roughly how far the source of the cosmic rays is from Earth.
With some clever statistical analysis, the team say some of the cosmic rays received by AMS-02 could well have come from dark matter annihilation, with the WIMP constituents having a mass of anywhere from 10 to 80 GeV — again fitting with the theory.
Though astronomers, cosmologists and particle physicists are working hard to crack the mystery of dark matter — if it even exists! — there is still a way to go. But experiments like this, where two entirely separate methods of data processing can lead to similar results, are very promising.
Perhaps the future isn’t so dark after all.
Image credit: NASA