When you look up on a clear, cloudless evening, the sky can appear positively ablaze with the light from billions of stars shining down upon us. However, it’s strange to think that there’s something else up there that accounts for far more of the Universe’s matter than all the stars and galaxies combined. This chapter is about something that cannot be seen, cannot be directly measured and while we know what it does, we don’t know what it is. ‘What on Earth is the matter?’ is an exasperated question inhabiting relationships worldwide – but when it comes to the Universe, it takes on a whole different meaning.

Dark matter accounts for around 27 per cent of the known mass of the Universe but it has never been directly observed. In fact, all the galaxies, stars and planets make up just 4.9 per cent of the Universe’s mass energy. So, if all the observable galaxies, stars and planets make up 4.9 per cent, and dark matter accounts for around 27 per cent, then what constitutes the rest of the Universe?1 Well, you can find out more about that in chapter E of my book DARK: An A to Z of the Cosmos. 

The ominously titled dark matter was given its name due to it not giving off, absorbing or reflecting light, rendering it invisible to the electromagnetic spectrum. However, despite this cloak of invisibility, there are some tell-tale signs that dark matter exists.

Similarly to black holes, evidence for dark matter includes its gravitational effects on visible matter, such as the movements and structure of the galaxies and also gravitational lensing, where light is bent

as it passes through the gravitational warping of space-time. Due to the fact that things that are dark absorb light, theoretical physicist Lisa Randall considers ‘transparent matter’ to be a more accurate description.2 Due to its reluctance to reveal itself while also showing its gravitational influence, I would like to propose ‘coy matter’ as a suitable alternative to help give this mysterious matter a much-needed PR overhaul. (There is probably a very good reason why nobody has asked me to officiate over the naming of important astrophysical anomalies.)

The first suggestion of dark matter came about in the 1930s through the separate observations of Jan Oort and Fritz Zwicky. In 1932 Jan Oort observed that stars in our galactic locality were moving too fast and thought that there must be more mass than was visible holding the galaxy together. He suggested that the missing mass might be due to darker stars that emitted too little light to be observed.3 A year later, Zwicky observed a cluster of around 1,000 galaxies called the Coma Cluster and made an estimation of the amount of mass contained within it. He also measured the velocities of some of the galaxies and, similarly to Oort, saw that they were moving faster than the total mass of the observable matter should be able to support.4

It wasn’t until the pioneering work of Vera Rubin in the 1960s and 1970s that the case for dark matter was taken seriously. Rubin’s studies of spiral galaxies found their outer edges to be travelling faster than expected – as fast as their centres. According to the observable mass within the galaxies, this quite simply shouldn’t be possible. Newtonian physics

dictates that the stars on the outside of a galaxy should be moving more slowly than those at the centre, which are feeling the effects of gravity more intensely.5 However, stars appear to move at roughly the same speed throughout galaxies, which means there must be way more mass in a galaxy than we can actually see, acting as a cosmic glue and holding them together. If the galaxies were only bound by the gravitational effects from their own observable mass (like planets and stars), they would have been pulled apart a long, long time ago. Rubin used the dark- matter conundrum as a metaphor for how well we understand the cosmos as a whole, commenting, ‘In a spiral galaxy, the ratio of dark-to-light matter is about a factor of ten. That’s probably a good number for the ratio of our ignorance-to-knowledge. We’re out of kindergarten, but only in about third grade.’6 Despite our knowledge of how important dark matter is to the composition of galaxies, it is still seemingly shrouded in cosmic camouflage. Common theories suggest that it may be WIMPs (Weak Interacting Massive Particles, hypothetically created early in the lifespan of the Universe) that interact with other forms of mass but neither give off nor absorb light.7 If this is the case, then it’s just a matter of time before detectors of sufficient sensitivity are developed to measure such incredibly puny interactions. Alternatively, it could well be that the current understanding of gravity is incorrect and what we currently assume is dark matter is a product of this misunderstanding. Other emerging theories refer to ‘supersymmetry and extra dimensions’8 – concepts that stray outside of the current standard model of physics and would require an author of considerably higher IQ to recount.

Bill Bryson succinctly commented in his book A Short History of Nearly Everything, ‘For the moment we might very well call them DUNNOS (for Dark Unknown Nonreflective Nondetectable Objects Somewhere.’9 For now, the prospect of directly observing dark matter or understanding its precise nature remains out of our grasp. But what we do know is that whether it’s an as-yet- undiscovered particle, or even an indication that our current understanding of gravity is incorrect, the unravelling of dark matter’s secrets will continue to challenge scientists for many years to come, and is another addition to the humble list of the Universe’s great unknowns.

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Image 1: Dark Matter illustration by Andreas Brooks from Dark (2023) by Jim Wilkins

Image 2: Ultra by Andreas Brooks → @brooksandreas

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