You might have seen a story or two claiming that astrophysicists have seen evidence of dark matter producing gamma rays. As exciting as it would be if true, we’re here to tell you that there are reasons to be skeptical. At least for now.
When we look out into space, light and gravity seem to be telling us conflicting things. The only light that we see, no matter where we look in the Universe, comes from the kind of matter that makes us up - the kind made out of protons, neutrons, electrons, and some of their more exotic cousins.
When light goes through a cloud of matter here on Earth, it emerges with distinct features that tell us about what kind of matter it went through. And when we check with telescopes, light universally has similar features, meaning that it’s only ever going through matter like it goes through stuff here on Earth.
There are some kinds of matter that don’t directly interact with light. Neutrons, for example, are electrically neutral, so they don’t really respond when light goes past them. But there are two other forces - the strong and weak nuclear forces - that do affect neutrons, which means we can use light to figure out if neutrons are there.
Gravity tells a different story. When we measure how much gravitation there is in the Universe, we find that there’s far more than the matter we can see could produce. If you go by gravity, the kind of matter that makes us up only accounts for something like a fifth of the gravity that’s out there. The rest is taken up by what has been called dark matter, since it doesn’t emit any light.
A leading contender for dark matter’s secret identity is a hypothetical class of particles that act a bit like more selective neutrons. They’re known as Weakly Interacting Massive Particles, or WIMPs.
WIMPs are like neutrons when it comes to light, but they also don’t do anything when the strong nuclear force is around. They only interact with other matter through gravity and the weak nuclear force, which explains why they’re so hard to find. If they exist, WIMPs would be more or less regular matter; they’re just matter that we’ve never been able to observe directly because of how they interact with the other stuff out there.
All matter in the Universe, whether dark or sparkling, is thought to have an antimatter counterpart. And when regular matter runs into its evil antimatter twin, the two disappear in a brief burst of gamma rays - the most energetic kind of light in the Universe. The same should happen to WIMPs, and it’s possible to predict what kinds of gamma rays would be produced in such a burst.
According to the authors of a new paper in the journal Physics of the Dark Universe, the Fermi telescope has seen gamma rays with just the right distribution and exactly the expected energy, indicating that evidence of WIMPs has been found.
While the gamma rays might be the first clear evidence of WIMPs, a couple of groups of scientists remain skeptical. Some have submitted papers saying that they’re not sure what has been producing the gamma rays, but there’s a fair chance it isn’t dark matter. One of these groups shares a member with the team claiming to have found dark matter, and all parties involved acknowledge that there are different ways to interpret the distribution of the gamma ray sources.
A group in the Netherlands has been much less agnostic, writing that a previously unseen collection of pulsars completely explains all of the gamma rays that were used as evidence of WIMPs. The authors of the dark matter paper claim that there is no reason to expect so many pulsars in that particular region of space, while the Netherlands group confidently disagrees.
The issue remains unresolved. Discovering that dark matter acts in another way like regular matter would rightfully be huge news; it would give us a much better idea of what dark matter actually is and suggest new ways to look for it.
As Carl Sagan would say, science is the marriage of two conflicting impulses: skepticism and wonder. We need to be open to new ideas while still questioning them; we need to seek answers without jumping to one too quickly.
More tests are needed to figure out where these gamma rays are coming from. For now, we'll just have to wait and see.