WHAT IS DARK MATTER?

It is a quintessential astronomical mystery: why does the Universe seem to contain more matter than we can see? Colin Stuart investigates.

Colin Stuart

If modern astronomy is to be believed, there is a mysterious substance filling the cosmos. Not only does it influence the Universe silently, but also passes invisibly and constantly through your body and the whole Earth: dark matter. One way to picture dark matter is as an invisible ‘glue’. Astronomers frequently find that they need an extra source of gravity to explain how cosmic structures remain bound together.

Take our own Milky Way Galaxy, the vast city of stars to which the Sun belongs. In the 1930s, the Dutch astronomer Jan Oort measured how stars near the Sun move through the Milky Way. He found something puzzling: the stars were moving too fast. So fast, in fact, that they should have been able to escape the Galaxy’s gravitational pull and drift away into space. But clearly they were not doing this.

Around the same time, the Swiss–American astronomer Fritz Zwicky observed a group of distant galaxies known as the Coma Cluster. When Zwicky measured the speeds of these galaxies, he found a similar problem. They were moving so rapidly that the cluster should have flown apart. Yet it remained intact. When Zwicky added up all the material he could see in the cluster, including stars, gas and dust, there was not nearly enough mass to provide the gravity needed to hold it together. He therefore proposed that some additional, unseen material must be present, supplying the ‘missing’ gravity. A native German speaker, Zwicky called it Dunkle Materie or dark matter.

Rotation curves

Despite the work of Oort and Zwicky, the idea of dark matter initially gained little traction. That changed in the 1970s, when the American astronomer Vera Rubin extended Oort’s studies to more than 50 spiral galaxies similar to the Milky Way. She showed that unusually fast- moving stars were not an exception, but a common and defining feature of such galaxies.

To understand why this was so troubling, astronomers had long assumed that spiral galaxies would behave like scaled-up versions of the Solar System. In the Solar System, planets orbit the Sun, and the farther away a planet is, the more slowly it moves, because the Sun’s gravity weakens with distance. When plotted on a graph, this gives what astronomers call a rotation curve.

Astronomers frequently find that they need an extra source of gravity to explain how cosmic structures remain bound together.

Most of a spiral galaxy’s visible mass lies in a central bulge, so stars were expected to orbit more slowly toward the galaxy’s edge. Rubin’s observations showed that this simply was not the case. The rotation curve did not decrease, instead it reached a peak and then remained stubbornly flat or even slightly increasing (see diagram).

One explanation is that while most of the visible matter is concentrated near the centre of a galaxy, most of the total mass is not. Instead, galaxies appear to be embedded within vast halos of invisible dark matter, with ordinary matter forming only a thin, luminous layer.

Testing dark matter

This idea can be tested. Astronomers can measure the mass of an object in space by studying how other objects orbit it. The Milky Way, for example, is surrounded by dwarf galaxies and ancient groups of stars called globular clusters. When astronomers use their motions to calculate the Galaxy’s mass, the result is striking: the Milky Way is about 15 to 20 times more massive than all its visible material combined. In other words, more than 90 per cent of its mass appears to be invisible, and this is usually attributed to dark matter.

Galaxy rotation is not the only phenomenon that points to an additional source of gravity being required. Another is gravitational lensing. Light travelling from a distant galaxy can be bent as it passes a massive cluster of galaxies, much like light passing through a glass lens. Astronomers regularly observe more bending than visible matter alone can account for, implying the presence of additional unseen mass or, more radically, a misunderstanding of the way that gravity truly works.

Another line of evidence comes from collisions between galaxy clusters. In these rare events, the hot gas within the clusters – which contains most of the ordinary matter – slows down and piles up at the centre of the collision. However, maps of the gravitational field, derived from lensing, show that most of the mass lies elsewhere. The Bullet Cluster is the most famous example. It is consistent with the expectations from dark matter. However, when the same technique is tried on different cluster collisions, the results are not as clear cut. So, a puzzle remains to be solved.

Without dark matter, galaxies and galaxy clusters may never have formed at all. Simulations of the early Universe show that ordinary matter alone clumps together too slowly. Only when dark matter is included do the simulations reproduce the large-scale structure of the modern Universe.

Dark matter’s identity

So what might dark matter be made of? For decades, the leading candidates were hypothetical particles known as WIMPs, or Weakly Interacting Massive Particles. Physicists have searched for them in deep underground laboratories, in space, and even at the South Pole.

One major experiment is IceCube in Antarctica. Although designed to detect neutrinos, IceCube can also test some dark matter theories. If WIMPs exist, they could accumulate in the Sun’s core and annihilate, producing neutrinos with distinctive energies.

So far, IceCube has detected no such signals.

Other experiments, including the Large Underground Xenon (LUX) detector and its successor LUX-ZEPLIN, have also come up empty. These experiments search for flashes of light produced when dark matter particles collide with atoms in tanks of liquid xenon.

There is the Alpha Magnetic Spectrometer (AMS-02), attached to the International Space Station. It too has joined the hunt by searching for particles that could be produced in dark matter collisions in the Milky Way. Although it has detected an excess of positron particles (the antimatter counterpart of an electron), these could just as plausibly originate from astrophysical sources as from dark matter.

The lack of evidence for WIMPs has therefore encouraged scientists to explore other possibilities. One alternative is the axion, an extremely light particle originally proposed to solve a problem in particle physics related to the way in which the strong nuclear force behaves. Axions could reveal themselves by converting into light in strong magnetic fields, which is what experiments such as Axion Dark Matter Experiment (ADMX) at the University of

Washington and Haloscope At Yale Sensitive To Axion CDM (HAYSTAC) are designed to detect. So far, they too have found nothing.

Alternatives to dark matter

With so many null results, some researchers have questioned whether dark matter exists at all. Instead, they wonder whether gravity works differently from Isaac Newton's description when it comes to very weak gravitational fields or on very large distance scales.

Of these approaches, the most well developed is called Modified Newtonian Dynamics, or MOND. It has had some remarkable successes in describing the rotation curve of galaxies without the action of dark matter by suggesting that there is a tiny but constant extra force of gravity that is hardwired into the fabric of the Universe.

While it is hard to deny that MOND does a better job of fitting such rotation curves than dark matter theories, there have been a few recent challenges. Some researchers claim that MOND cannot reproduce the observed effects of widely separated binary star systems. However, online claims that the theory has been falsified beyond doubt are certainly premature. After all, the same claims could be made for dark matter in light of its failure to show up in any experiment so far.

With so many null results, some researchers have questioned whether dark matter exists at all.

Meanwhile, the search for dark matter continues. While it is simple to think of it as a single type of particle, there is nothing that says it has to be. Multiple effects or particles could be contributing to the phenomenon.

Until astronomers can determine exactly what is going on, however, the true nature of most of the Universe’s matter will remain one of science’s greatest unanswered questions.

Colin Stuart is an astronomy author and speaker. Join his free astronomy club at colinstuart.substack.com