Dark Sector 1: Dark Matter

After decades of research, astronomers and cosmologists have determined that we don’t understand more than 95% of the contents of the Universe. Less than 5% (one twentieth) is comprised of the same kind of matter as us - made of atoms and their component protons, electrons, neutrons and other subatomic particles. Despite our colossal ignorance of the remainder, we can constrain some of its properties, and crucially have determined that it comes in two flavours, dubbed dark matter and dark energy. Collectively known as the dark sector, this vast amount of invisible content lies at the cutting edge of astrophysics and has captured the imagination of both science fiction readers and writers.

In the first of two entries, I’m going to take a look at the enigma that is dark matter [1].

The invisible glue of the Universe

As telescopes improved in the second half of the twentieth century, measurements of stars in our own galaxy and others became ever more detailed. The existence of galaxies and clusters of galaxies beyond our own was confirmed. The structure of the Milky Way was traced out and the relative motion of stars began to be measured. One thing became rapidly clear - both the galaxies in clusters and the stars at the outskirts of individual galaxies were moving far faster than would be expected of them.

Galaxies like our own are formed of a flat disk of stars and gas, orbiting around a concentrated bulge of stars and light towards their centre. Like a toy on a string, swung rapidly around, their motion pushes them outwards, while the tug of the string constantly pulls them back on a curving path. Twirl the toy too fast, or use too fine a thread and the string will snap, unable to hold against the outwards force. In the same way stars in the outskirts of galaxies should fly outwards, unless held in place by a strong ‘string’ of gravity connecting them to the centre of mass. As early as the mid-1930s, Fritz Zwicky had noted that ‘dark’ matter must exceed luminous material by a substantial margin in order to explain the implied gravitational force. Follow up work, and particularly observations by Einasto et al (1974) and Rubin et al (1978) revealed that all the gravity possible from the stars we could measure by their light was unable to hold our galaxy together: there was a “Missing Mass Problem”.

This, together with further clues from other large scale structure led to a single conclusion which, by the early 1980s, was widely accepted by the astronomical community [2]: there was some form of matter out there that not only didn’t shine, but didn’t interact with heat or light in any measurable way (so it remained invisible to astronomers everywhere from gamma rays through to infrared and radio waves). The only way we could measure this substance was through its gravitational force. What’s more, results from a mixture of particle physics and the cosmic microwave background radiation made it clear that this matter wasn’t baryonic - it couldn’t be made of atomic components like our own without violating all sorts of physics. As a result, it was dubbed dark matter both for its lack of visibility and for our own lack of knowledge regarding it.

The Mystery of Dark Matter

The missing mass problem, as one of the biggest unknowns in astronomy, naturally caught the eye of science fiction audiences.

One of the earliest authors to pick up on this dilemma was Frederik Pohl. His Heechee series began with the novel Gateway (1977) which described human attempts to retroengineer technology left by a mysterious ancient race. The final discussions in his sequel, Beyond the Blue Horizon (1980) introduced the idea that the long-vanished Heechee race was associated with a solution to the missing mass problem. This was shown to arise from the Heechee’s ability to manipulate gravitational mass. In other words, mass and galaxy dynamics failed to match not because of extra mass that was unseen (the dark matter explanation) but because mass had been removed and the anomalous motion was in the process of changing (i.e. a deliberate alien manipulation of cosmology). This theme continued to recur in the third novel, Heechee Rendezvous (1984) and later.

A more philosophical use of dark matter, now referred to by that name and concept, appeared in Andrew Weiner’s short story Waves (Asimov’s, March 1987). This narrative is set in an America in which leisure and arts flourish on government dole following an economic crash. The protagonist is one of many men in his community entranced by a mysterious singer. This woman was formerly an astronomer and part of the team which confirmed the expected end date of the Universe - a consequence of excessive amounts of matter in the Universe is that it might result in gravity eventually pulling everything back to a singularity in a Big Crunch, and this is the scenario here.

As the narrator character explains:

“Dark matter was the realm of cosmic shadows, the invisible empire beneath the thin veil of ordinary matter. Lurking there always, pushing and pulling the visible universe this way and that.”

However after universities were closed and the hard sciences collapsed, during the crash, the woman physicist turned to psychology and then began a singer, exploring the darkness and cold inside human minds and their interaction with the nature of reality. Drawing parallels between this cold darkness and the forces which shape the human mind forms the core of the story.

Philosophy and its relation to dark matter also formed a surprising core to the short story Gnostic Endings by Barrington J Bayley (Interzone, July 1994). Here Earth finds itself alone in a universe empty of life. With typical human determination, humanity plans a seeding programme, sending out spacecraft to fill the empty seas (and ultimately land) of Earth-like worlds with life. A group of activists share the belief of the ancient gnostic cults of the Middle East, believing that the confinement of souls to physical matter harms them and results from a cosmic mistake. Understandably, given this belief, they attempt to disrupt the seeding, partially winning over one of the crew. As he considers the premise, dark matter is mentioned as a deeply negative part of physical reality and evidence of a failed creation:

 “The swirling galaxies were aborted residues. Most of their mass was dark matter, sub-suns which had failed to ignite. Dead relics of a divine mistake, revolving in empty grandeur.”

In keeping with its philosophical themes, the story ends with multiple possible options or layers, leaving room for interpretation by the reader.

While these early stories tended to focus on the mystery and enigma of dark matter, the existence of this substance became normalised through the 1990s, and became familiar through documentaries and articles, including a number of science-fact articles appearing in science fiction magazines. This growing public familiarity with the concept was reflected in the normalisation of dark matter as part of the background of science fiction, as, for example, in appearances of dark matter asteroids and nebulae in the television series Star Trek: The Next Generation (e.g. TV episode “In Theory”, 1991), Star Trek: Voyager (e.g. TV episodes “Cathexis”, “One Small Step”, both 1999) and Star Trek: Enterprise (e.g. TV episode “First Flight”, 2003). These typically cause gravitational distortions, with no electromagnetic radiation to warn of their presence, and with unexpected consequences for the ships involved.

Dark Matter Technology

Given that we don’t understand the origin or properties of dark matter, an interesting avenue of speculation in science fiction is the possibility that, better understood, it might prove the basis of new technologies or physical effects.

The comedic animated series Futurama (TV series, 1999-2003, and ongoing sequels) follows the adventures of a hapless delivery guy from the USA of the twentieth century who is refrigerated for a millennium before joining a chaotic delivery company of the thirtieth century. The Planet Express delivery spaceship is fuelled by dark matter pellets, burnt in an engine invented by Professor Farnsworth. These are ultra-dense objects of initially mysterious origin… which, in true Futurama style, later prove to be the excreta of a cute and cuddly form of alien (TV Episode “Love Labours Lost in Space”, 1999).

Futurama is far from serious in its detailed handling of physics, but plays with concepts both of the dangers of unwise or unethical experimentation and the opportunities opened by scientific innovation.

A more serious exploration of potential dark matter based technology can be found in Robert Reed’s 2003 novel Sister Alice, which was based on five shorter pieces originally published in Isaac Asimov’s SF Magazine between 1993 and 2000. Set in a very distant future in which people are essentially immortal, Sister Alice describes a culture in which one thousand god-like “families”, each comprising many clones of an original founder, possess highly advanced technologies which are prevented from falling into the hands of the rest of humanity for the sake of peace and stability (with the consent of the entire society). The titular character is one of the most ancient and powerful of her family, and one of a number of her peers who experiment with the idea of making bubble universes into which humanity can expand. Unfortunately their prototype universe erupts into our own, causing colossal damage to the core of the Milky Way and, in the fallout of that event, significantly changing the life of the youngest of the cloned family, a boy called Ord.

The technology (and how it was initially developed) is never described in any detail, but uses ‘dark matter’, occasionally described as ‘shadow matter’ [3] - in fact the part of Alice (or her peers) made of normal, baryonic matter is only a tiny fraction of her true extent, and many parts of her are in the form of detachable devices with which her original body was enhanced. These dark matter devices are largely invisible to observers, but are tangible and can be touched, and also detected as large masses:

 “Natural dark matter didn’t exist in this form: coagulated, tangible, capable of interacting with visible matter. But with sufficient energies and the proper cleverness, it was possible to make the wild particles behave, making them cling to one another and play games with a baryonic boy. These were great technologies, and Ord knew little beyond that.”

The consequences of such large dark matter masses, in terms of an individual’s true mass affecting her surroundings gravitationally, are somewhat confusing. Usually the large mass of 'talents' is left in a distant orbit somewhere to avoid damaging a given world, but it’s not clear how the individual makes use of such talents remotely and instantaneously. However the mass of an individual’s dark matter can also be detected and used in itself as a weapon. Indeed much of the application of dark matter technologies in this narrative is likened to magic and falls under Clarke’s third law as advanced beyond our ability to rationalise.

As dark matter has percolated through popular culture, further dark matter weapons or defensive technologies have become common in science fictional combat games and narratives, including tabletop roleplaying examples like Warhammer 40,000. The term is more common still as a label on what are otherwise apparently normal, baryonic devices, as seen for example in the Gundam anime universe.

Aliens made of Dark Matter

Perhaps the most common use of dark matter in science fiction, however, is to imagine alien life made of this substance. While we don’t know what dark matter is, and can only interact with it through gravity, we cannot rule out that it interacts with itself through forces and fields which act to it as electromagnetism does to us, holding together dark matter atoms, molecules and perhaps even cells. Just as dark matter is impervious to electromagnetic fields (including the photons of light), we might be just as impervious to any equivalent fields, allowing an entire sphere of materials to be colocated with us, without our knowledge.

The novel Sister Alice touched on this possibility, with at least one world and its biosphere being constructed from dark matter and thus spared from the devastation occurring to baryonic matter all around it (as a result of the bubble-universe experiment). Creating this took a god-like level of technology and an equally immense and powerful mind, suggesting that in the novel’s universe, at least, natural dark matter life is unlikely.

By contrast, hard science fiction writer Stephen Baxter has made extensive use of natural dark matter life in his Xeelee sequence of novels. These place a young humanity in a universe shaped by the actions of a much older and more powerful species, the Xeelee, whose motivations are initially beyond comprehension. In the 1994 novel Ring, Baxter makes it clear that the Xeelee are, in fact, at war with an equally mysterious enemy [4]. 

As Baxter explains, the photino is the hypothetical partner of the photon under a (real world) scientific theory known as supersymmetry, in which every particle may have a supersymmetric equivalent. These ‘sparticles’ would manifest as a class of candidate dark matter known as WIMPS - weakly interacting massive particles. Unlike classical dark matter, these are not solely gravitationally interacting and will occasionally react to a photon or baryon, but interactions with baryonic (normal matter) particles are exceptionally rare:

“Dark matter and baryonic matter attracted each other gravitationally. Dark matter particles could interact with baryonic matter through other forces: but only feebly, and in conditions of the highest density - such as at the heart of stars. In Earthlike conditions, the worlds of baryonic and dark matter slid through and past each other, all but unaware, like colonies of ghosts from different millenia.”

In the Xeelee sequence, and as revealed in Ring, the 'Photino Birds' are made of their eponymous dark matter particle in the same way we are made of protons, neutrons and electrons. And while their dark matter realm hardly interacts with our own, their occasional rare but non-zero interactions with other forces are necessary to their existence. This has led to them disrupting the hydrogen burning of our sun - in fact they prove to reproduce in suns, using ephemeral baryonic structures in these dense environments to help shape their photino offspring. However the photino birds appear to be using their interactions deliberately to ensure stars collapse quickly to a more stable (and thus good for photino-life) white dwarf state, through causing accelerated energy-loss and hence death of stars,

“The birds didn’t need spectacular, blazing stars. They certainly didn’t need instability, novae and supernovae, the disruption of dying stars. All they demanded from a star was a stable gravity well and a trickle-source of proton-photino interaction energy”

Understandably this affects the universe at large in the narrative, and will lead to a dead galaxy in an astronomical blink-of-an-eye. The titular Ring is a cosmic string megastructure designed as the Xeelee response to this attack on the existence of baryonic life.

Star Trek: Voyager (TV series, 1996-2001) also picked up on the possibility of dark matter life in the 2000 episode “Good Shepherd” in which the captain’s attempts to encourage underperforming crew members are hampered by the discovery of a dark matter life-form, which (for the purposes of television) they contrive to make visible. Similar dark matter aliens have appeared in novels including Starplex (Robert Sawyer, 1996) and The Alice Encounter (John Gribbin, 2014). As in the Xeelee novels, the dark matter entities described here can pass through normal (baryonic) matter and often remain unaware of it. The materials we describe as solid are comprised of minute subatomic particles that only occupy a fraction of the volume of an atom, which in turn only occupies a fraction of the volume in a liquid or crystal. The remainder of the volume is filled with powerful electromagnetic fields which other matter cannot penetrate and so is perceived as solid. In the absence of such fields, as for a dark matter being insensitive to them, passing through a dense region of matter might be akin to walking through a snow storm, while normal regions would feel more like walking through atmosphere to us, with no perceptible sense of pressure or opacity. Similarly, here on Earth, we constantly pass through a moderate density of dark matter (which comprises about half the total local matter density) without being aware of it, but can detect the impact of its mass and gravity on the orbit of the Sun around the Milky Way.

Also common to these fictions is the idea that when baryonic and dark matter entities do become aware of one another, hostility is a virtual inevitability. Unusually, given the history of warfare as we know it, this is seldom because of a conflict for resources - one form of matter is unlikely to be of significant value to the other except in rare near-magical cases of advanced technology such as Sister Alice. Instead conflict arises through the distortion of space-time by gravity and through basic xenophobia and misunderstandings.

A more benign form of dark matter alien can be found in the short story Utriusque Cosmi by Robert Charles Wilson (2009, reprinted in Clarke’s World magazine in 2014). The plot follows a woman, Carlotta, transported into a conscious non-organic cloud called the Fleet as an escape from the Earth, which in the process of destruction. Over an extended time period, the Fleet rescues other species from the many worlds that are destroyed by a mysterious and poorly-understood phenomenon which appears to be alien in origin. It is not until towards the end of the story, when the protagonist faces her own imminent destruction, that it is revealed that the devastation has been caused by dark matter entities largely indifferent to the imperceptible life of normal matter but nonetheless damaging to it:

“The logic of it is this: Life-bearing worlds generate civilizations that eventually begin playing with dark matter, posing a potential threat to the continuity of the Old Ones. Some number of these intrusions can be tolerated and contained—like the Fleet, they were often an enriching presence—but too many of them would endanger the stability of the system. It’s as if we were germs, girl, wiped out by a giant’s immune system. They couldn’t see us, except as a somatic threat. Simple as that.” 

However eventually the dark matter entities prove to be embarked on the same rescue-before-destruction mission as the Fleet, but on a larger scale which might well enable originally-human consciousnesses including Carlotta to outlive the universe itself. The rescue within a rescue structure proves reminiscent of the ancient conception of the world as embraced by ever larger and more mysterious shells on which the planets and stars were fixed - the utriusque cosmi of the title [5].

Dark Matters

Dark matter is now very much part of our understanding of the universe. While we do not know what it is, we can measure and quantify its effects, making it as real as electricity and magnetism were in the years before Maxwell and others developed electromagnetic theory. The advent of modern space telescopes has allowed still more varied evidence to be acquired, as, for example, in the case of the Bullet Cluster. In this case, a collision between two groups of galaxies has resulted in the bulk of the normal (electromagnetically visible) matter (shown in pink in the NASA image here) being displaced from the bulk of the mass (dark matter, shown in blue) which was inferred from gravitational effects.

As a result of this accumulation of many avenues of evidence, we now know just how minor a fraction of the Universe’s contents matter like ourselves comprises. As Baxter puts it, “The glowing baryonic matter was a mere glittering froth on the surface of that dark ocean”. However, attempts to explain the nature of dark matter remain frustrating.

While the supersymmetry theory used by Baxter in Ring was a leading contender for several decades, observations within the last decade (from astrophysics, gravitational wave detectors and from particle physics colliders) have ruled out the most elegant and simplest forms of this paradigm. While supersymmetric particles may exist, their properties would now have to fit into a narrow and harder-to-explain window in energies, and so much of the simplicity of the original theory is lost. At the same time, observations of gravitational lensing in the Milky Way’s halo have largely ruled out the presence of massive compact halo objects (MACHOs, named in contrast to weakly interacting massive particles or WIMPs). These were hypothesized large objects of non-luminous matter (perhaps, but not necessarily, including black holes or failed stars).

With supersymmetry becoming disfavoured, leading contenders for dark matter have become axions (a theorized particle beyond the standard model of particle physics) or primordial black holes (tiny defects in space-time left by the Big Bang, which did not require baryonic structures like stars to collapse to form them). However no single current theory is both testable and able to explain all the observed phenomena. 

Despite this, and as its use in SF demonstrates, the existence of dark matter has slowly drifted from the philosophical questions of its earliest days to a more tangible and normalised component of our understanding of the Universe. While the majority of SF audiences will have little idea of the theories proposed to explain dark matter, the majority will now have heard of it and be comfortable with the idea of dark matter structures such as those appearing in twenty-first century fiction such as Sister Alice or recent versions of Star Trek.

Ironically, from a scientific perspective, such structures may be one of the least likely forms of dark matter. Without electromagnetic radiation, it’s challenging for most theorized forms of dark matter to radiate away its gravitational energy and collapse to solid forms. Instead it remains dynamically-hot, with rapid random motion acting as a form of outward pressure. This would explain why dark matter appears to remain in vast halos around galaxies, rather than collapsing into their cores - but it also means that forming dark matter stars, planets and even life-forms is likely impossible.

Science fiction nonetheless fossilizes a snap-shot of both the scientific understanding of dark matter in a given time and the manners in which the ideas of science can be expanded upon, explored and extrapolated. It has captured the journey of dark matter from an obscure, intangible concept to a very real substance that makes up the vast majority of matter in the Universe we inhabit. The creativity of authors, as well as that of scientists working in this field, will no doubt continue to shape new ideas and explore new evidence as it arises.