Dark matter is a real pain in the neck.
The term dark matter itself refers to a hypothetical substance that seems to solely interact with the rest of the universe via gravity and to serve as the scaffolding for galaxies and other massive cosmic structures. Actual particles of dark matter have never been found, however—despite decades of intensive effort to discover them. Some critics consequently dismiss the concept as a mere fudge factor that physicists use to prop up their incomplete theories of how the universe works. But whether dark matter is a “real” thing or a helpful figment of theorists’ imaginations, there’s simply too much evidence to just wish that the problem would go away.
Call it what you will, there’s obviously something very strange going on in the universe. Stars at the outskirts of galaxies orbit far too quickly. Galaxies buzz around in clusters much too fast. Matter-rich strands of the cosmic web coalesce too swiftly. And there are so many more examples.
On supporting science journalism
If you’re enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
Every attempt to relegate dark matter to the basement of discarded physics ideas, such as modifying the force of gravity to accommodate all the associated observational quirks, has failed—hard. While it’s impossible to rule out such approaches—you never know what a clever theorist might cook up tomorrow—a half-century of cultivation has yet to yield any satisfying fruit.
Decades of evidence have largely ruled out the obvious candidates for dark matter that have been inspired by high-energy physics. But high-energy physics isn’t the only game in town. There are other fields, including condensed matter physics, the branch of physics that examines the properties of large collections of matter, such as how all the atoms in a pane of glass conspire to make it transparent. This field of physics has its own strange corners, such as the weird world of superconductivity. And these corners are strange enough to provide some potentially useful inspiration for understanding the puzzle of dark matter.
Our best guess as to the nature of dark matter is that there’s some form of matter that does not interact with light—or really much of anything else or even itself. (Admittedly, this is not that great of a guess, but it’s the best one we’ve got.) This matter takes up the bulk of almost every galaxy and larger structure, and it just, well, sits there, existing, making itself known only through its gravitational pulls on visible matter.
Every line of cosmological inquiry points to the humbling realization that only a slim fraction of the matter in the universe lights up. After centuries of effort, developing the periodic table, the particle zoo, the Standard Model of particle physics, the forces of nature and all the rest, we now know that we’ve barely scratched the surface.
But science is a journey only for the humble, so we have but one choice in front of us: onward.
We have many powerful tools at our disposal in our journeys through the dark corners of the universe. One tool is our suite of observations, measurements taken at scales from the galactic to the cosmic that span the breadth of the observable universe and the depth of deep time. All of these observations inform, and ultimately judge, any candidate theory. We may be in the dark about what dark matter is, but we have a very good sense of what it does. If you have your own idea to explain dark matter, then it must come through the crucible of observations intact. If an idea fails anywhere along the way, we move on and try the next one.
The other powerful tool is physics itself, our mathematical exploration of the world. We don’t fully understand dark matter, its identity or characteristics or interactions with the rest of the universe. But we know, to varying degrees of confidence, what the rest of the universe is up to. Dark matter is like a missing piece in a puzzle; we don’t know what the piece is, but we roughly know the shape it has to take.
Whatever dark matter is, it must obey the laws of physics (even if we do not yet know all those laws).
For example, when the universe was less than a minute old, dark matter must have somehow disconnected from normal matter (a process called “freezing out”) to get the right present-day amount we infer from observations. This is how we came up with our leading candidate for dark matter, the WIMP, or weakly interacting massive particle. We had some hypothetical particles generated in theories of physics that, if they were active and abundant and generally around in the universe, would have naturally done exactly that.
But we have yet to directly detect a WIMP, and its theoretical underpinnings have been shown to be on thin ice.
So onward we go.
There’s also the axion, another hypothetical particle sourced from high-energy physics and one that is trillions of times lighter than the WIMP. If the axion exists and has the right properties, it, too, could do all the things that we know we need dark matter to do.
But we have yet to directly detect an axion—although, to be fair, we haven’t searched for axions nearly as extensively as we have for WIMPs, simply because WIMPs were thought to be a shoo-in for dark matter.
So onward we go.
In May Guanming Liang and Robert Caldwell, both at Dartmouth College, published a paper in Physical Review Letters in which they offered their own candidate for dark matter. A pessimist might look at this study and roll their eyes: “Oh, joy, yet another proposal for a candidate particle, probably the umpteenth one this month alone, that is almost certainly wrong—just another random stab in the dark, another strand of spaghetti thrown against the fridge of cosmology.”
That response would be fair. This model is probably—no, almost certainly—wrong. But that’s because most models are wrong most of the time. If we knew the answer ahead of time, we wouldn’t need to be doing science. We can only find the right answer by sifting through all the wrong ones, picking the wheat from the chaff, trying again and again until we find something worthy.
But we only know when we try.
And admirably, Liang and Caldwell’s model not only tries but tries something truly new. Instead of drawing inspiration from high-energy physics, with hypothetical particles derived from this or that exotic interaction, the authors look to condensed matter physics and especially the bizarre nature of superconductivity.
In a regular conductor, electrons carry electricity, but they also offer resistance. At low enough temperatures and in the right materials, however, the electrons condense—or, if you will, freeze out—arranging themselves in pairs in a lower-energy configuration. This eliminates electrical resistance and makes the magic of superconductivity happen.
By analogy, the Liang and Caldwell model views dark matter as a soup of exotic particles born mere moments after the big bang, in the bizarre era before protons and neutrons arrived. This soup doesn’t interact with normal matter but also doesn’t necessarily have any mass on its own. As the cosmos expands and cools, the dark matter particles condense out and clump up, forming massive “droplets” that go on to have their own separate evolution, disconnected from the rest of the visible matter in the universe except through their gravitational influence.
The calculations are complex and tentative but promising. The main advantage of this approach is that it allows for a new mechanism to create dark matter in the heady conditions in the first few minutes after the big bang that isn’t dependent on the same steps that the usual WIMP procedure follows. And this model doesn’t just recapitulate existing dark matter evolution. If the exotic particles have some mass, then only some of them condense out to form dark matter. The rest get locked in place as a background that saturates the universe, potentially playing the role of dark energy, the mysterious force that appears to be accelerating the expansion of the universe. Crucially, in this model, dark energy can vary with time, which aligns with tentative results coming out of galaxy surveys.
Studies like these are only the first step: a plausibility check that gets roughly the right amount of dark matter at roughly the right time. It still remains to be seen if this can account for the absolute mountain of evidence that we have for dark matter: Can it simultaneously explain the broad spectrum of behaviors we attribute to dark matter, from the earliest epochs of the cosmos to the modern star-filled universe?
And can it pass the ultimate test of all? Can it predict the existence of a particle—or a condensation droplet of dark matter—that we could someday directly see for ourselves?
The search for the true nature of dark matter is frustrating indeed because the hurdles any model has to clear are numerous, to say the least. We won’t believe this model, or any other hypothesis, until it can succeed where so many others have failed.
So onward we go.