To put it simply, the entire visible universe is huge. In the scheme of it, we really are just tiny dots on a floating rock, in a vast and constantly expanding cosmos. 

Yet, as it turns out, that’s not even close to the full story. All the visible objects, planets and galaxies contribute less than 15% of the mass in the universe. 

The other 85%? Nobody knows for certain, but it has a name.

Dark matter.

More can be said about what dark matter is not, than what it is. It isn’t the baryonic or “normal” matter such as protons, neutrons and electrons which comprise our visible world. It also isn’t antimatter, composed of subatomic particles with opposite charges to normal matter. Instead, dark matter interacts with normal matter in a manner entirely different to that of antimatter. It’s not a type of black hole, nor simply a form of radiation, or a type of massless particle.

So, what can be conclusively said? Essentially, nothing.

As the name suggests, dark matter emits no light and therefore is not visible in the way normal matter is, making it difficult to observe. In fact, dark matter has only been “observed” by way of its gravitational effects. Therefore, we know it must have mass in order to be able to interact with visible matter gravitationally. It’s also imperative for it to be big enough to cause the massive gravitational effects seen in galaxies (Lochner et al., 2005).

Estimates place the mass-energy content of the cosmos as being composed of 26.8% dark matter, 68.3% dark energy and a relatively miniscule 4.9% normal matter (Greicius, 2013). The terms dark matter and dark energy are often thrown around somewhat interchangeably. However, they explain distinct aspects of observed gravitational and physical phenomena. Dark matter can be thought of as an invisible substance which is only seen through its effects on gravity - the unexplained gravitational forces that hold together rapidly rotating galaxies and stopping them from flying apart. Dark energy is then the force responsible for pushing these clusters of galaxies and the universe apart, accelerating the rate of expansion of the universe (NASA/WMAP Science Team, 2013).

Given the lack of answers about what dark matter is, an interesting question to ponder is how its existence was even discovered.

Swiss astronomer Fritz Zwicky was the first to propose the idea of “dark matter”. His observations of the Cloma galaxy cluster led him to suggest if individual galaxies within the cluster were only held together by the gravitational force of visible mass, the galaxies should fly apart due to their high velocity (American Museum of Natural History, 2000). He termed this mysterious force responsible for binding galaxy clusters together “dark matter”. It wasn’t until the 1970s that Vera Rubin became the first person to establish the existence of dark matter through her work with spiral galaxies. 

Spiral galaxies aren’t stationary. They rotate, with stars different distances from the centre moving in roughly circular orbits around this centre. The highest concentration of visible stars is found within the core region of a galaxy, leading to the assumption that the majority of mass, and therefore gravity, should also be concentrated there. An implication of this is the expectation that the farther a star is from this gravitational centre of a galaxy, the slower its projected orbital speed should be (American Museum of Natural History, 2000). However, alongside astronomer Kent Ford, Rubin made the puzzling observation that stars in both the centre and outer regions of any galaxy were moving at the same speed (American Museum of Natural History, 2000). Her calculations provided convincing observational evidence of Zwicky’s theory. The presence of a significant mass of invisible matter in the outer regions of a galaxy would create an even, spherical distribution of matter, gravitationally explaining the observed rotation of galaxies and their velocity distribution (NASA/WMAP Science Team, 2013).

Fifty years later and experimental evidence still remains the only “proof” of dark matter we have, having been unable to directly detect dark matter.

Despite this, a majority of scientists are confident in its existence. Rubin’s insight into the velocity distribution of galaxy rotation curves is amongst some of the most convincing observational evidence for the presence of dark matter. Also supporting its existence are the various discrepancies that arise in the process of gravitational lensing.

Gravitational lensing occurs when an emitted source of light is deflected or distorted by the gravitational field of a large mass. Based upon the degree of deflection, the gravitational potential of the object can be calculated, alongside the amount of matter in the lensing object  (Xenon Dark Matter Project, 2022). Yet, the strength of this gravitational lensing observed in many galaxy clusters is significantly greater than that calculated from visible matter alone. These inconsistencies point to the existence of unseen mass, or dark matter, as a convincing explanation for the observed lensing effects.

It’s become clear that the standard model of physics, explaining the different particles and forces comprising the visible world, cannot be used in attempting to explain dark matter. In response, researchers are exploring a number of avenues to find hypothetical new particles. Amongst the most likely candidates for the composition of dark matter are two classes of particles: Weakly Interacting Massive Particles (WIMPs) and axions. 

WIMPs are distinguished as a class of particles created thermally in the early universe at very high temperatures, while axions originate predominantly from non-thermal mechanisms (Griest, 2002). Compared to WIMPS, or other known type of particles, axions would be thousands of times lighter but also significantly more abundant than WIMPs (Darling & Knight, 2022). Given the infinite potential to invent hypothetical substances that resolve the enigma of dark matter, experimentation to find these particles has significant challenges. 

Current research efforts are focused on the detection of such particles. 

More than a kilometre underground in Stawell, Victoria, the Stawell Gold Mine has been converted into an underground laboratory – one with no light, no noise, and no radioactivity to interfere with dark matter signals (Lippincott, 2023). Here, an experiment known as DAMA/Libra, which started in Italy in 1998, is being replicated. For two decades, what is suspected to be dark matter has been detected at the same time each year in Italy. The Stawell Lab is seeking to verify these results, operating below the equator to determine any potential effect of seasonal interference from the Earth (Darling & Knight, 2022). The research utilises the technology SABRE (Sodium iodide with Active Background REjection), which are sodium iodide crystals that emit flashes of light if a sub-atomic particle hits the nuclei of atoms within the crystals (Darling & Knight, 2022). Hence, if a particle of dark matter hits a nucleus, a tiny flash of light should be created.

Simultaneously, researchers at the University of Western Australia have been working on the detection project ORGAN (Oscillating Resonant Group Axion), in order to determine the presence of axions (McAllister, 2022). Despite not having detected any dark matter signals thus far, such experimentation has still offered important insights. Not detecting dark matter within a certain mass range and level of sensitivity allows exclusion limits to be set around the possible characteristics of axions. This tells researchers where to stop looking and, instead, where they should be focusing their resources and efforts.

Despite the disarray around “solving” the conundrum of dark matter, alongside its less than reassuring name, it’s not actually something that people should be scared about. The gravity that dark matter is responsible for enables our existence, with dark energy having allowed the expansion of the early universe to become what we see, and don’t see, today (Xenon Dark Matter Project, 2022). 

Detecting the presence of dark matter is about advancing our understanding of the size, structure, and future of the universe. Current research approaches may seem slightly haphazard, attempting to find something that has never been detected and may not even exist. But when pursuing strange cosmological phenomena beyond our understanding, taking a wild stab in the dark may be exactly what we need to do.

References

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American Museum of Natural History (2000). Vera Rubin and Dark Matter. Retrieved

September 1, 2023 from https://www.amnh.org/learn-teach/curriculum-collections/cosmic-horizons-book/vera-rubin-dark-matter

Darling, A., & Knight, B. (August 20, 2022). The search for dark matter. ABC News.

https://www.abc.net.au/news/2022-08-21/dark-matter-particle-physics-sabre-experiment-stawell-victoria/101113010

Greicius, T. (March 21, 2013). Planck Mission Brings Universe Into Sharp Focus. NASA.

https://www.nasa.gov/mission_pages/planck/news/planck20130321.html

Griest, K. (2002). WIMPs and MACHOs. In P. Murdin (Ed.), Encylopedia of Astronomy and

Astrophysics: CRC Press.

Lippincott, H. (August 9, 2023). Researchers dig deep underground in hopes of finally

observing dark matter. The Conversation. https://theconversation.com/researchers-dig-deep-underground-in-hopes-of-finally-observing-dark-matter-211075

Lochner, J. C., Williamson, L., & Fitzhugh, E. (2005). Possibilities for Dark Matter. Retrieved

August 29, 2023 from https://imagine.gsfc.nasa.gov/educators/galaxies/imagine/titlepage.html

McAllister, B. (July 26, 2022). This Australian experiment is on the hunt for an elusive

particle that could help unlock the mystery of dark matter. The Conversation.  https://theconversation.com/this-australian-experiment-is-on-the-hunt-for-an-elusive-particle-that-could-help-unlock-the-mystery-of-dark-matter-187014

NASA/WMAP Science Team. (2013). WMAP produces new results. Retrieved September

13, 2023 from https://map.gsfc.nasa.gov/news/

Xenon Dark Matter Project. (2022). Dark Matter. Retrieved August 25, 2023

from https://xenonexperiment.org/partners/