Of all the sciences, physics is unique in that it can come to a broad consensus with math alone—very little tactile evidence required. Never has this been the case more than with dark matter. Though it's far more abundant than the matter we can see and feel, dark matter as we know it is virtually immune to observation, making it elusive and tricky to understand. So, how do we know what it is, and how are scientists continuing to study it?
How Does Dark Matter Differ From Regular Matter?
Matter—the visible kind, that is—interacts with the universe in many ways. It absorbs and, in many cases, emits electromagnetic radiation in the form of gamma rays, visible light, infrared, and more. It can generate magnetic fields of various sorts and strengths. Matter has mass, creating the force of gravity, the effects of which can be readily observed.
All these things make studying matter convenient, especially regarding its interactions with light. Even a black hole, which emits no light, blocks light by sucking it in. But what if the light coming from behind a black hole simply passed right through and on into our telescope lenses? How would we ever have proven the existence of a black hole, in that case?
That's the situation physicists face with dark matter. Dark matter does not seem to interact with the universal electromagnetic field in the slightest—that is, it does not absorb or emit light of any kind. Dark matter seems only to interact with the universe as we can observe it through a single physical force: gravity. So, in the case of our invisible black hole, we might have been able to notice it by seeing how the light coming to us from a certain section of the sky was bent relative to our expectations, knocked slightly off course by passing close to an object bending the surface of the spacetime it's traversing. Adding up enough light-bending observations, scientists could probably figure out the position and even mass of the invisible singularity.
However, dark matter is harder to study than even that because it does not come conveniently clumped into super-dense balls like stars and black holes. (That would be far too easy.) Instead, the primary theory of dark matter says that it is made of hypothetical particles called Weakly Interacting Massive Particles (WIMPs), which are about as well understood as their catch-all name implies. WIMPs don't even seem to interact with each other through anything more than gravity, meaning dark matter does not fuse to form larger or more complex molecules and remains in a simple—and highly diffuse—gas-like state.
Thus, dark matter's gravitational impact is extremely spread out and, it turns out, can only be observed when we look at the large-scale distribution of visible matter in the universe: things like galactic superclusters and the corresponding super-voids. It's theorized that after the Big Bang, the properties of dark matter would have led it to settle down far more quickly than regular matter, going from a uniform gas cloud to a somewhat clumped network of smaller clouds and connecting tendrils. As these tendrils stretched across the universe, their distribution may have directed where regular matter eventually collected, helping to shape where and how galaxies formed.
Not only is dark matter invisible, but the effects of its gravitational potential are so physically sprawling that they're hard to measure. The light from a single star won't be measurably bent by dark matter when it reaches us, as it was in passing our invisible black hole; that light might very well have originated, traveled through, and arrived all within the reach of a single universal super-thread of invisible dark matter.
When Was Dark Matter First Proposed?
Gravity affects everything, at all scales, according to the same basic formulae. So, as scientists started to examine the universe on a larger scale, they noticed these gravity formulae delivered increasingly wrong predictions.
As early as the 1930s, Fritz Zwicky discovered that galaxies in the Coma cluster were moving as though they were subject to far more gravitational force than could be explained through a simple accounting of the normal matter we could see. Decades later, Vera Rubin famously noted that stars in spiral galaxies rotate around the galactic center far faster than they ought to, leading to later studies showing that spiral galaxies must be made up of about six times as much dark mass as the regular kind.
But the compelling evidence didn't come until the advent of techniques like weak gravitational lensing and the ability to read the cosmic microwave background (CMB) radiation. In effect, gravitational lensing produces an extremely large-scale version of watching light bend around our invisible black hole. It gets around the scale issue with even more scale, watching how the collected light from billions of clustered stars bends as it travels across large fractions of the diameter of the known universe. Several increasingly accurate CMB maps made from the 1960s onward confirmed similar discrepancies in the movement of mass early in the universe's history.
What Have We Learned Since Then?
Scientists have gotten creative in their attempts to track down dark matter. (With dark matter, you have to be; the field is full of frustrating dead ends and answers that only lead to more questions.) This has led to some pretty interesting results over the past few years.
Some researchers have theorized that the "sterile neutrino"—a hypothetical, neutrally charged particle with no mass—could make up the "stuff" of dark matter. In 2014, physicists thought they'd detected a 3.5 kiloelectron volt signal (keV) coming from deep within the Milky Way. This signal was believed to be consistent with what would arise from decaying sterile neutrino dark matter. In 2020, equipped with new data from the XMM-Newton space X-ray telescope, another team of physicists set out to replicate that signal—and never found it. This doesn't necessarily mean sterile neutrinos don't exist or aren't involved with dark matter; it just means there's more observational work and analysis to be done in that particular arena.
Remember the WIMPs from earlier? In 2023, researchers at the University of Hong Kong attempted to nail down whether dark matter was made from WIMPs or axions, another hypothetical particle. Using gravitational lensing and computer modeling, they found that simulated dark matter composed of axions aligned more with our working understanding of light and gravity than simulated dark matter composed of WIMPs. This still doesn't guarantee dark matter is made up of axions, though; it just means axions might be an essential piece of the puzzle.
In Germany, scientists are trying to observe dark matter in a bold new way: by creating it. ALPS II, an experiment at the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, consists of a 250-meter tunnel with an optical cavity. This cavity amplifies a laser that is exposed to an intense magnetic field produced by 12 superconducting magnets as it travels down the tunnel. It's thought that such a powerful magnetic field might turn a photon into an axion, thus revealing the hypothetical particle to be real.
Researchers are trying to tackle dark matter from all sides to uncover whatever information they can. But why is dark matter so important? If it's so evasive, why bother studying it at all?
Why Dark Matter Matters
For the most part, humanity's collective research says something very much like the modern conception of dark matter has to exist. Calculating exactly how much of this "something" would be necessary to fill in the gaps has produced some fairly mind-boggling figures. By modern estimates, the universe is only about 5% regular matter and energy and about 27% dark matter, or more than five times as much. The remaining 68% of the universe is thought to be dark energy—a topic for another day.
The fact that so much of our universe consists of dark matter makes the stuff worth studying. But by figuring out dark matter, we can also expect to gain a better understanding of our galaxy and the distribution of the cosmos outside it. Our universe hasn't just been impacted by dark matter; it's been defined by that impact. As we touched on earlier, the Milky Way owes its existence, shape, and place to the early gravitational influence of dark matter. And who wouldn't want to know about the architect behind our galactic home?
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