What Is Dark Matter? The Science of Galaxy Glue Explained

Introduction

Dark matter is something that very little is actually known about; it is a topic that many have speculated about but none have explained completely. The term "dark matter" was first used by Fritz Zwicky in the 1930s to describe the matter that makes up most of what is believed to be the mass of the universe (Hossenfelde and McGaugh). In other words, Zwicky looked to the skies and tried to make sense of the seemingly all-prevalent matter that does not emit, absorb, or reflect light at all. In fact, the only way anyone knows dark matter exists is through its effects: gravitational pull, radiation, and the overall structure of the universe itself, which suggests that dark matter must be present. With that said, the best that anyone has been able to do to date is hypothesize about what this glue that seems to hold the universe together actually is, why it is dark, and what secrets it contains. This paper reviews the subject and what research on it has shown so far, as well as what scientists are hoping to find out about dark matter in the years ahead.

Background and Key Terminology

The basic concept of dark matter is this: it cannot be viewed by a telescope. It is invisible to the human eye because it does not interact with light at all (Rubin). The only reason Zwicky was able to assert it existed was because he made an inference based on what he believed to be gravitational effects: he surmised that galaxies far away rotated at such slow speeds that more mass must be present in them than the eye could see. Zwicky suspected that if this extra mass were not present, galaxies would fly apart due to their rotational speed. Thus, dark matter in his hypothesis was essentially a type of galaxy glue.

Dark matter has to be defined in such generic terms as "galaxy glue" because no one really knows exactly what it is — or even whether it truly exists. It has none of the make-up of other things that people can see and observe: it shares no particles with stars or planets (Clegg). Scientists have a Standard Model of particle physics, but dark matter simply does not fit into that model. Some researchers therefore think dark matter could consist of new particles that have not yet been discovered or understood. For that reason, scientists continue to research the field and seek a better understanding (Clegg).

Since dark matter is undetectable by the naked eye and largely indescribable outside of speculative theory, it is helpful to be familiar with some of the key terms used in discussions of the topic.

First, there is galaxy: a collection of stars, gas, dust, and dark matter, bound together by gravity (Clegg). Gravity is another important term — although not scientifically defined as a force, it is often described in terms of having force on other objects. It is theoretically defined as the thing that attracts two bodies with mass, keeps planets in orbit around stars, and governs the large-scale structure of the universe (Rubin). Gravity is often taken for granted as something that verifiably exists — but it is actually still a theory rather than a law. There are in fact other explanations for why things fall, why things float, and why things rise — such as the theory of ether (Boersma).

Another useful term, though also theoretical, is Weakly Interacting Massive Particles (WIMPs). WIMPs are hypothetical types of dark matter particles, and some researchers speculate that they explain what dark matter is (Clegg). Gravitational lensing is another term often used in dark matter research. It refers to the bending of light from a distant source, such as a galaxy, by the gravitational field of an intervening object, such as another galaxy (Granata et al.). This effect is used to map dark matter, and the Hubble Space Telescope plays a key role in that work.

In all, the vocabulary needed to thoroughly discuss dark matter is quite extensive, but these terms provide a sufficient starting point.

Recent Work in Dark Matter Research

Some of the most significant recent work on dark matter is conducted at the Large Hadron Collider (LHC), the world's largest and most powerful particle collider, located at CERN. It is used in experiments that could provide insights into dark matter (CERN). CERN notes that "the Standard Model is a collection of theories that embodies all of our current understanding of fundamental particles and forces… [however], although the Standard Model is a very powerful theory, some of the phenomena recently observed — such as dark matter and the absence of antimatter in the universe — remain unexplained and cannot be accounted for in the model" (CERN 6). Thus, the work that CERN is doing in trying to collide particles to better understand energy is also important for determining whether an understanding of dark matter particles can be obtained.

Lisa Randall, writing for Nature, describes dark matter as "an elusive substance that permeates the universe, exerts many detectable gravitational influences, yet eludes direct detection." Randall argues that dark matter should really be called transparent matter, since light simply passes through it. Like many other scientists, Randall hopes that the Large Hadron Collider at CERN might eventually detect dark matter particles through its experiments.

Recent work such as the Xenon1T experiment — the world's most sensitive dark matter detector at the time — has broken some ground in trying to explain dark matter further. In 2020, the Xenon1T collaboration reported what could be a sign of new physics, such as the existence of a WIMP (Aprile et al.). However, the signal could also be due to a previously unaccounted-for background process. Like so much other work on dark matter, nothing has proven conclusive.

The Hubble Space Telescope continues to be used to gather data on dark matter through observations of gravitational lensing, which helps map the distribution of dark matter in galaxy clusters (Granata et al.). Experiments like the Axion Dark Matter Experiment (ADMX) are specifically designed to detect axions, which are particles theorized to be even lighter than WIMPs (Chadha-Day et al.). Nothing conclusive has emerged from this work either — but scientists continue to discuss the topic and develop new experimental approaches.

The evidence relating to axions, WIMPs, and dark matter itself remains entirely speculative and theoretical — as, it must be remembered, gravity itself is still a theory. Most people would respond to such a reminder with surprise, since gravity has been accepted by much of the public as a verifiable fact. But there are and have been other explanations for the physics of the universe. Dark matter is of interest to scientists because, if properly understood, it could help explain some of the bigger mysteries visible to the eye and the telescope — such as what keeps galaxies from breaking up, what holds them together, and what truly underlies the structure of the universe.

For that reason, it is helpful to keep an open mind about the universe and the topic of dark matter, since nothing conclusive has been presented as evidence to date. Research is based on theory and speculation, with articles like that of Chadha-Day et al. being used to put forward new ideas — for instance, their proposal that axions constitute dark matter. These ideas have yet to be verified and validated. It is much like any other field where researchers try to know what they do not know and cannot firmly verify exists in the first place. There is almost a religious or faith-based quality to it in the long run — people try to prove or disprove the existence of God in much the same way, it often seems.

Ultimately, what researchers are doing is trying to understand a mystery by applying the logic and reasoning at their disposal in the most coherent way possible. This is a natural part of the scientific method and should not be viewed as a fault. The fault, if there ever is one, is found in the premise or starting point of the argument — the assumptions made when setting out. If those assumptions are wrong, everything that follows can lead inquiry in a very wrong direction.

Conclusion

In conclusion, dark matter — for now — is best understood as the glue that holds the galaxy together. What it actually is, or how to define it precisely, remains an open question. Scientists are pursuing answers through various models, experiments, and detection methods, including particle collision experiments at CERN. New technologies have already emerged from this work that could benefit different industries and fields. Whether we are any closer to unraveling the mysteries of the universe is harder to say. Perhaps this is why, in the Middle Ages, people were willing to accept that the glue holding the galaxy together was God — and simply left it at that.

Works Cited

Aprile, Elena, et al. "Search for inelastic scattering of WIMP dark matter in XENON1T." Physical Review D 103.6 (2021): 063028.

Boersma, Geert. "Gravity, inertia and ether." Physics Essays 27.2 (2014): 259–266.

CERN. CERN Brochure 2021-004-Eng. https://cds.cern.ch/record/2809109/files/CERN-Brochure-2021-004-Eng.pdf

Chadha-Day, Francesca, John Ellis, and David J. E. Marsh. "Axion dark matter: What is it and why now?" Science Advances 8.8 (2022): eabj3618.

Clegg, Brian. Dark Matter and Dark Energy: The Hidden 95% of the Universe. Icon Books, 2019.

Granata, Giovanni, et al. "Improved strong lensing modelling of galaxy clusters using the Fundamental Plane: Detailed mapping of the baryonic and dark matter mass distribution of Abell S1063." Astronomy & Astrophysics 659 (2022): A24.

Hossenfelder, Sabine, and Stacy S. McGaugh. "Is Dark Matter Real?" Scientific American 319.2 (2018): 36–43.

Randall, Lisa. "What is Dark Matter?" Nature (2018). https://www.nature.com/articles/d41586-018-05096-y

Rubin, Vera. "Dark matter in the universe." Scientific American 1 (1998): 106–110.