*are we blissfully sat in Plato’s cave only looking forward? should I do a post about Plato’s cave next?

Imagine looking up at the night sky, studded with countless stars, galaxies swirling in the cosmic dark. It feels vast, almost infinite. Yet, astronomers tell us that everything we can see – every star, planet, nebula, and galaxy – makes up less than 5% of the total stuff in the universe. The rest, a staggering 95%, is invisible, composed of two mysterious components we call dark matter and dark energy. This isn’t science fiction; it’s one of the biggest puzzles in modern cosmology, a hunt for the hidden scaffolding and the strange propulsive force that shape our universe. Understanding what these dark constituents are, and how we’re trying to detect them, is fundamental to grasping the true nature of reality, the past, and the ultimate fate of the cosmos. It’s a quest that pushes the boundaries of our knowledge and technology.

The story of dark matter begins not with a deliberate search, but with a puzzling observation. Back in the 1930s, a Swiss astronomer named Fritz Zwicky was studying the Coma Cluster, a massive collection of galaxies [1]. He measured how fast the individual galaxies were whizzing around within the cluster. He then estimated the cluster’s total mass based on the light emitted by all its galaxies – essentially, counting the stars. What he found was deeply strange: the galaxies were moving far too quickly. Based on the visible mass, the gravity should have been far too weak to hold the cluster together; the galaxies should have flown off into space long ago. Zwicky proposed that there must be a huge amount of unseen matter – “dunkle Materie” or dark matter – providing the extra gravitational pull needed to keep the cluster intact. His calculations suggested there might be hundreds of times more dark matter than visible matter. At the time, his ideas were largely dismissed, perhaps partly due to limited observational data and Zwicky’s own somewhat abrasive personality.

The idea languished for decades until the 1970s, when the work of American astronomer Vera Rubin and her colleague Kent Ford provided compelling new evidence [2]. They were meticulously measuring the rotation speeds of stars within spiral galaxies, including our neighbour, Andromeda. According to Newtonian gravity and our understanding of how mass is distributed based on light, stars further from the galactic centre should orbit more slowly, much like outer planets in our solar system orbit the Sun more slowly than inner planets. But Rubin and Ford found something completely different: the stars’ speeds stayed remarkably constant, or ‘flat’, even at great distances from the centre. The only way to explain this was if the galaxies were embedded in massive, invisible halos of dark matter, extending far beyond the visible stars and providing extra gravity. Rubin’s careful, undeniable data forced the astronomical community to take dark matter seriously. As she later remarked, “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 average. In the universe, it’s closer to a factor of six” [3]. These observations, combined with Zwicky’s earlier work on clusters, formed the observational bedrock for the existence of dark matter.

So, what could this invisible stuff actually be? We know it interacts gravitationally – that’s how we know it’s there – but it doesn’t seem to interact with light (or any electromagnetic radiation) in any significant way, hence why it’s ‘dark’. It also doesn’t appear to interact strongly with ordinary matter, otherwise, we’d likely have bumped into it already. This rules out ordinary matter like faint stars, dust clouds, or rogue planets, which collectively fall under the banner of ‘baryonic’ matter (the protons and neutrons that make us up). While some dark matter could theoretically be baryonic objects that are just hard to see (like Massive Astrophysical Compact Halo Objects, or MACHOs – think brown dwarfs or isolated black holes), extensive searches have shown these cannot account for the vast majority of the missing mass [4]. The leading hypothesis is that dark matter is composed of some new type of fundamental particle, something not included in the Standard Model of particle physics, which describes all known fundamental particles and forces (except gravity).

One of the most popular candidates for this particle is the WIMP, or Weakly Interacting Massive Particle. As the name suggests, WIMPs are thought to be relatively heavy particles that interact only via gravity and the weak nuclear force (the force responsible for certain types of radioactive decay). This ‘weakly interacting’ nature explains why they are so hard to detect directly. Billions of WIMPs could be streaming through you, and the Earth, every second without leaving a trace. However, very occasionally, a WIMP might collide with the nucleus of an atom in a detector, causing it to recoil like a subatomic billiard ball. Detecting this tiny recoil is the goal of ‘direct detection’ experiments. These experiments are typically located deep underground – in mines or tunnels – to shield them from cosmic rays and other background radiation that could mimic a WIMP signal. Examples include the LUX-ZEPLIN (LZ) experiment in South Dakota, USA [5], the XENONnT experiment under the Gran Sasso mountain in Italy [6], and PandaX-4T in China’s JinPing underground laboratory. These experiments often use large tanks filled with liquefied noble gases like xenon or argon, or sometimes supercooled crystals. They employ incredibly sensitive detectors to look for the faint flash of light or tiny charge released when a nucleus recoils. Despite enormous effort and increasing sensitivity, no definitive WIMP signal has been found yet.

Another promising candidate is the axion. Axions are hypothetical particles that are incredibly light, potentially billions or trillions of times lighter than an electron. They were originally proposed in the 1970s to solve a theoretical problem in particle physics related to the strong nuclear force (the ‘strong CP problem’) [7]. It was later realised that if axions exist and were produced in the early universe, they could also constitute dark matter. Unlike WIMPs, axions are expected to interact extremely weakly, but they might have one exploitable property: in the presence of a very strong magnetic field, an axion could theoretically convert into a photon (a particle of light) [8]. Experiments like the Axion Dark Matter eXperiment (ADMX) in the US use powerful magnets and sensitive microwave receivers to essentially ‘listen’ for the faint hum of photons produced by axions converting within the magnetic field [9]. Like WIMP searches, these experiments are incredibly challenging, requiring exquisite sensitivity and noise reduction, and have yet to yield a confirmed detection. Other less mainstream, but still plausible, candidates include sterile neutrinos (hypothetical heavier cousins of the known neutrinos) and even primordial black holes (black holes formed in the very early universe, before stars existed).

Besides trying to catch dark matter particles directly, scientists also employ ‘indirect detection’ methods. If dark matter particles are WIMPs, they might occasionally collide and annihilate each other, especially where dark matter is thought to be densest, like the centre of our galaxy or in dwarf galaxies orbiting the Milky Way. These annihilations could produce showers of recognisable particles, such as high-energy gamma rays, neutrinos, or antimatter particles (like positrons), which we could then detect with telescopes. Space-based telescopes like the Fermi Gamma-ray Space Telescope scan the skies for excess gamma rays from promising regions [10]. Experiments like the Alpha Magnetic Spectrometer (AMS-02) on the International Space Station search for an excess of antimatter particles in cosmic rays [11]. On Earth, huge neutrino detectors like IceCube, buried deep in the Antarctic ice, look for high-energy neutrinos that might originate from dark matter annihilations in the Sun or the galactic centre [12]. So far, while some intriguing hints have emerged, none have provided conclusive proof of dark matter annihilation. A third approach involves trying to create dark matter particles in high-energy collisions at particle accelerators like the Large Hadron Collider (LHC) at CERN [13]. If dark matter particles can be produced, they would likely escape the detectors unseen (due to their weak interactions), but they would carry away energy and momentum. Physicists look for events where there is a significant imbalance – ‘missing energy’ – that could signal the production of invisible dark matter particles. Again, despite extensive searches, no such signal has been definitively observed beyond the predictions of the Standard Model.

While the hunt for dark matter focuses on an invisible substance holding things together, the search for dark energy involves an invisible force pushing everything apart. In the late 1990s, two independent teams of astronomers were using Type Ia supernovae – exploding white dwarf stars with a remarkably consistent peak brightness – as ‘standard candles’ to measure the expansion rate of the universe at different times in cosmic history. They expected to find that the expansion was slowing down due to the gravitational pull of all the matter in the universe. Instead, they made a revolutionary discovery: the expansion of the universe is actually accelerating [14, 15]. It was as if you threw a ball into the air, and instead of slowing down and falling back, it started speeding up and flying away faster and faster. This discovery earned the team leaders – Saul Perlmutter, Brian Schmidt, and Adam Riess – the 2011 Nobel Prize in Physics.

This acceleration implies that there must be some form of energy permeating space itself, acting counter to gravity on cosmic scales, pushing spacetime apart. This mysterious component was dubbed ‘dark energy’. According to our best measurements, based on supernovae, the cosmic microwave background (the faint afterglow of the Big Bang), and the large-scale distribution of galaxies, dark energy makes up about 68% of the total energy density of the universe [16]. Dark matter accounts for about 27%, leaving less than 5% for all the ordinary matter we know.

What could dark energy be? The simplest explanation, and the one that currently fits the data best, is that it’s the ‘cosmological constant’, represented by the Greek letter Lambda (Λ) in Einstein’s equations of general relativity. Einstein originally introduced this term into his equations in 1917 to allow for a static universe (which was the prevailing view at the time), effectively acting as an anti-gravity force to balance the pull of matter [17]. When Edwin Hubble discovered the universe was expanding in 1929, Einstein reportedly called the cosmological constant his “biggest blunder”. Ironically, it seems he might have been right all along, but for the wrong reason – not to keep the universe static, but to explain its accelerating expansion. Physically, the cosmological constant could represent the intrinsic energy of empty space itself – ‘vacuum energy’ – arising from quantum fluctuations where particles constantly pop in and out of existence. However, theoretical calculations of how much vacuum energy there should be based on quantum physics predict a value that is astronomically larger – about 120 orders of magnitude larger! – than the value observed for dark energy [18]. This staggering discrepancy, known as the ‘cosmological constant problem’, is one of the deepest mysteries in physics. As cosmologist Michael Turner put it, “The number one problem in physics is dark energy. It’s related to the destiny of the Universe. It’s related to the nature of spacetime” [19].

Because of this problem, alternative theories exist. Perhaps dark energy isn’t constant but is instead a dynamic field that changes over time and space, often referred to as ‘quintessence’ [20]. Or maybe Einstein’s theory of general relativity itself needs modification on cosmological scales, and the acceleration is a sign that gravity behaves differently over vast distances than we currently understand.

Unlike dark matter, we don’t expect to ‘detect’ dark energy in a laboratory. Instead, we study its effects on the expansion history and the growth of structure in the universe. Astronomers use various techniques. They continue to refine measurements using Type Ia supernovae. They study the subtle patterns in the cosmic microwave background radiation, which encodes information about the universe’s composition and geometry. Another powerful technique involves measuring Baryon Acoustic Oscillations (BAO) [21]. These are like frozen ‘sound waves’ from the very early universe that left a characteristic imprint on the distribution of galaxies, providing a ‘standard ruler’ to measure cosmic distances and the expansion rate. Large astronomical surveys are key to this effort. Projects like the Dark Energy Survey (DES) have already mapped hundreds of millions of galaxies [22]. Future observatories like the Vera C. Rubin Observatory currently under construction in Chile [23], and space missions like the European Space Agency’s Euclid telescope (launched in 2023) [24] and NASA’s Nancy Grace Roman Space Telescope (planned for launch later this decade) [25], will map billions of galaxies over huge swathes of the sky with unprecedented precision. By tracking how the distribution of galaxies and the expansion rate have changed over cosmic time, these surveys aim to pin down the properties of dark energy – is it constant (like Lambda) or does it evolve? – and test whether general relativity holds true on the largest scales.

The continued elusiveness of both dark matter and dark energy presents a profound challenge. The fact that decades of searching for WIMPs and other favoured dark matter candidates have yielded nothing has led some physicists to question the underlying assumptions and explore alternative particle candidates or even modifications to gravity, such as Modified Newtonian Dynamics (MOND) [26], although MOND struggles to explain all the evidence attributed to dark matter, particularly on cluster scales and in the CMB. The cosmological constant problem associated with dark energy remains a major theoretical headache. Furthermore, there’s a growing ‘Hubble tension’ – different methods of measuring the current expansion rate of the universe (the Hubble constant) are yielding slightly different results, potentially hinting at new physics beyond our current Lambda-CDM model (the standard cosmological model incorporating cold dark matter and a cosmological constant) [27]. These controversies highlight that while the standard model of cosmology is remarkably successful in explaining a vast range of observations, it’s built on two pillars – dark matter and dark energy – whose fundamental nature remains completely unknown.

The quest to understand the dark universe is far more than just an astronomical curiosity. Discovering the identity of dark matter would revolutionise particle physics, revealing new fundamental particles and possibly new forces beyond the Standard Model. Unravelling the nature of dark energy could transform our understanding of gravity, spacetime, and the ultimate fate of the cosmos. Will the universe continue to accelerate its expansion forever, leading to a cold, empty ‘Big Rip’ or ‘Big Freeze’, or could dark energy change its behaviour in the future? Finding these answers requires pushing the boundaries of experimental sensitivity, observational precision, and theoretical ingenuity. We stand at a fascinating juncture, knowing that 95% of our universe is hidden from view, a vast cosmic ocean whose shores we have only just begun to explore. What wonders, or perhaps even stranger realities, lie waiting in those dark depths?

References and Further Reading:

1. Zwicky, F. (1933). Die Rotverschiebung von extragalaktischen Nebeln. Helvetica Physica Acta, 6, 110–127. (Historical paper noting the ‘missing mass’ in the Coma Cluster).
2. Rubin, V. C., & Ford, W. K., Jr. (1970). Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions. The Astrophysical Journal, 159, 379. (Seminal paper on flat galaxy rotation curves).
3. Quote attributed to Vera Rubin, widely cited in articles about her work, e.g., Discover Magazine interview (2002) or similar profiles. Exact original source can be tricky to pin down definitively but reflects her findings.
4. Alcock, C., et al. (MACHO Collaboration). (2000). The MACHO Project: Microlensing Results from 5.7 Years of LMC Observations. The Astrophysical Journal, 542(1), 281–307. (Example of MACHO search results).
5. LUX-ZEPLIN (LZ) Experiment Website. Sanford Underground Research Facility. https://lz.lbl.gov/ (Provides information on the LZ direct detection experiment).
6. XENON Collaboration Website. https://xenonexperiment.org/ (Provides information on the XENONnT direct detection experiment).
7. Peccei, R. D., & Quinn, H. R. (1977). CP Conservation in the Presence of Pseudoparticles. Physical Review Letters, 38(25), 1440–1443. (Original theoretical paper proposing the mechanism leading to axions).
8. Sikivie, P. (1983). Experimental Tests of the “Invisible” Axion. Physical Review Letters, 51(16), 1415–1417. (Paper outlining the principle of axion-photon conversion).
9. Axion Dark Matter eXperiment (ADMX) Website. University of Washington. https://www.phys.washington.edu/groups/admx/home.html (Provides information on the ADMX experiment).
10. Fermi Gamma-ray Space Telescope Website. NASA. https://fermi.gsfc.nasa.gov/ (Information on the telescope used for indirect detection searches).
11. Alpha Magnetic Spectrometer (AMS) Experiment Website. https://ams02.space/ (Information on the ISS-based particle detector).
12. IceCube Neutrino Observatory Website. University of Wisconsin–Madison. https://icecube.wisc.edu/ (Information on the South Pole neutrino detector).
13. CERN Website – Exploring the Secrets of the Universe. https://home.cern/science/physics/dark-matter (Overview of dark matter searches at the LHC).
14. Riess, A. G., et al. (Supernova Search Team). (1998). Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant. The Astronomical Journal, 116(3), 1009–1038.
15. Perlmutter, S., et al. (Supernova Cosmology Project). (1999). Measurements of Omega and Lambda from 42 High-Redshift Supernovae. The Astrophysical Journal, 517(2), 565–586.
16. Planck Collaboration. (2020). Planck 2018 results. VI. Cosmological parameters. Astronomy & Astrophysics, 641, A6. (Definitive measurements of cosmological parameters from the Planck satellite).
17. Einstein, A. (1917). Kosmologische Betrachtungen zur allgemeinen Relativitätstheorie (Cosmological Considerations in the General Theory of Relativity). Sitzungsberichte der Preussischen Akademie der Wissenschaften, part 1, 142–152.
18. Weinberg, S. (1989). The cosmological constant problem. Reviews of Modern Physics, 61(1), 1–23. (Classic review article on the problem).
19. Quote from Michael S. Turner, often cited in discussions on dark energy, e.g., University of Chicago news articles or public lectures.
20. Caldwell, R. R., Dave, R., & Steinhardt, P. J. (1998). Cosmological Imprint of an Energy Component with General Equation of State. Physical Review Letters, 80(8), 1582–1585. (Early paper discussing quintessence models).
21. Eisenstein, D. J., et al. (SDSS Collaboration). (2005). Detection of the Baryon Acoustic Peak in the Large-Scale Correlation Function of SDSS Luminous Red Galaxies. The Astrophysical Journal, 633(2), 560–574. (Seminal paper on BAO detection).
22. Dark Energy Survey (DES) Website. https://www.darkenergysurvey.org/ (Information on the DES project).
23. Vera C. Rubin Observatory Website. https://www.lsst.org/ (Information on the upcoming observatory).
24. Euclid Consortium Website. https://www.euclid-ec.org/ (Information on the ESA Euclid mission).
25. Nancy Grace Roman Space Telescope Website. NASA. https://roman.gsfc.nasa.gov/ (Information on the upcoming NASA mission).
26. Milgrom, M. (1983). A modification of the Newtonian dynamics as a possible alternative to the hidden mass hypothesis. The Astrophysical Journal, 270, 365–370. (Original paper proposing MOND).
27. Riess, A. G., et al. (2022). A Comprehensive Measurement of the Local Value of the Hubble Constant with 1 km/s/Mpc Uncertainty from the Hubble Space Telescope and the SH0ES Team. The Astrophysical Journal Letters, 934(1), L7. (Example paper highlighting the Hubble tension).
28. Bertone, G., & Hooper, D. (2018). History of dark matter. Reviews of Modern Physics, 90(4), 045002. (A comprehensive review article).
29. Frieman, J. A., Turner, M. S., & Huterer, D. (2008). Dark Energy and the Accelerating Universe. Annual Review of Astronomy and Astrophysics, 46, 385–432. (A detailed review of dark energy).
30. NASA Science Universe Website – Dark Energy, Dark Matter. https://science.nasa.gov/universe/dark-energy-dark-matter/ (Accessible overview from NASA).

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