Dark matter – one of the greatest mysteries of modern astrophysics and cosmology. Although it makes up the majority of the Universe's matter, its nature remains unclear. Dark matter neither emits, absorbs, nor reflects light at observable levels, so it is "invisible" (English “dark”) to telescopes relying on electromagnetic radiation. Nevertheless, its gravitational effect on galaxies, galaxy clusters, and the large-scale structure of the Universe is undeniable.
In this article, we will discuss:
- Historical hints and early observations
- Evidence from galaxy rotation curves and clusters
- Cosmological and gravitational lensing data
- Candidates for dark matter particles
- Experimental search methods: direct, indirect, and accelerators
- Selected questions and future prospects
1. Historical hints and early observations
1.1 Fritz Zwicky and the missing mass (1930s)
The first serious hint of dark matter was given by Fritz Zwicky in the 1930s. Studying the Coma galaxy cluster, Zwicky measured the velocities of cluster members and applied the virial theorem (which relates the average kinetic energy of a bound system to its potential energy). He found that galaxies were moving so fast that the cluster should have dispersed if it contained only the mass of stars and gas we can see. To keep the cluster gravitationally bound, a lot of "missing mass" was needed, which Zwicky called "Dunkle Materie" (German for "dark matter") [1].
Conclusion: Galaxy clusters contain much more mass than is visible – indicating the existence of a huge invisible component.
1.2 Early skepticism
For many decades, some astrophysicists cautiously considered the idea of vast amounts of non-luminous matter. Some leaned towards alternative explanations, such as abundant clusters of faint stars or other dim objects, or even modifications to the laws of gravity. However, as evidence accumulated, dark matter became one of the foundations of cosmology.
2. Evidence from Galaxy Rotation Curves and Clusters
2.1 Vera Rubin and Galaxy Rotation Curves
A crucial breakthrough occurred in the 1970s and 1980s when Vera Rubin and Kent Ford measured rotation curves of spiral galaxies, including the Andromeda galaxy (M31) [2]. According to Newtonian dynamics, stars far from the galaxy center should move more slowly if most of the mass is concentrated in the central bulge (core) region. However, Rubin found that star rotation speeds remained constant or even increased well beyond the visible extent of the galaxy's matter.
Implication: Galaxy environments contain extended halos of “invisible” matter. These flat rotation curves strongly reinforced the theory that a dominant, non-luminous mass component exists.
2.2 Galaxy Clusters and the “Bullet Cluster”
Additional evidence comes from studies of galaxy cluster dynamics. Beyond the previously studied Coma cluster by Zwicky, modern measurements show that the mass determined from galaxy velocities and X-ray emission data also exceeds the visible matter alone. A particularly striking example is the Bullet Cluster (1E 0657–56), observed during a collision of galaxy clusters. Here, the mass determined by lensing (from gravitational lensing) is clearly separated from the bulk of the hot, X-ray emitting gas (ordinary matter). This separation is strong evidence that dark matter is a distinct component, different from baryonic matter [3].
3. Cosmological and Gravitational Lensing Evidence
3.1 Formation of Large-Scale Structures
Cosmological simulations show that in the early Universe there were slight density perturbations – visible in the cosmic microwave background (CMB). These perturbations grew over time into the vast network of galaxies and clusters we observe today. Cold dark matter (CDM) – non-relativistic particles that can clump under gravity – plays a crucial role in accelerating structure formation [4]. Without dark matter, it would be very difficult to explain the large-scale structures formed within the available time since the Big Bang.
3.2 Gravitational Lensing
According to the General Theory of Relativity, mass warps spacetime, causing light passing nearby to bend. Gravitational lensing measurements – both of individual galaxies and massive clusters – consistently show that the total gravitational mass is much greater than the matter that emits light alone. By studying distortions of background sources, astronomers can reconstruct the true mass distribution, often detecting extensive invisible mass halos [5].
4. Candidates for Dark Matter Particles
4.1 WIMP (weakly interacting massive particles)
Historically, the most popular class of dark matter particles has been WIMPs. These hypothetical particles are thought to:
- are massive (typically in the GeV–TeV range),
- are stable (or extremely long-lived),
- interact only gravitationally and possibly via the weak nuclear force.
WIMP particles conveniently explain how dark matter could have formed in the early Universe with the correct relic density – due to the so-called "thermal freeze-out" process, when, as the Universe expands and cools, interactions with ordinary matter become too rare to significantly annihilate or alter the abundance of such particles.
4.2 Axions
Another interesting candidate is axions, originally proposed to solve the "strong CP problem" in quantum chromodynamics (QCD). Axions would be light, pseudoscalar particles that could have been produced in the early Universe in sufficient quantities to make up all the required dark matter. "Axion-like particles" are a broader category that can arise in various theoretical frameworks, including string theory [6].
4.3 Other candidates
- Sterile neutrinos: heavier neutrino variants that do not interact via the weak force.
- Primordial black holes (PBH): hypothesized black holes formed in the very early Universe.
- "Warm" dark matter (WDM): particles lighter than WIMPs that could explain some small-scale structure discrepancies.
4.4 Modified gravity?
Some scientists propose modifications of gravity, such as MOND (modified Newtonian dynamics) or other more general theories (e.g., TeVeS), to avoid exotic new particles. However, the "Bullet Cluster" and other gravitational lensing data show that true dark matter – which can be separated from ordinary matter – explains observations much better.
5. Experimental searches: direct, indirect, and accelerators
5.1 Direct detection experiments
- Goal: to capture rare collisions of dark matter particles with atomic nuclei in highly sensitive detectors, usually located deep underground to shield from cosmic rays.
- Examples: XENONnT, LZ, and PandaX (xenon detectors); SuperCDMS (semiconductor).
- Status: so far there is no conclusive signal, but the sensitivity of experiments is reaching ever lower interaction cross-section limits.
5.2 Indirect detection
- Goal: to search for dark matter annihilation or decay products – e.g., gamma rays, neutrinos, or positrons – where dark matter is densest (e.g., Galactic center).
- Instruments: Fermi Gamma-ray Space Telescope, AMS (Alpha Magnetic Spectrometer on the ISS), HESS, IceCube, and others.
- Status: several intriguing signals have been observed (e.g., GeV gamma-ray excess near the Galactic center), but so far not confirmed as evidence of dark matter.
5.3 Accelerator studies
- Goal: to create possible dark matter particles (e.g., WIMPs) through high-energy collisions (e.g., proton collisions at the Large Hadron Collider).
- Method: searching for events with large missing transverse energy (MET), which could indicate invisible particles.
- Result: so far, no confirmed new physics signal compatible with WIMP has been found.
6. Unanswered questions and future prospects
Although gravitational data undoubtedly indicate the existence of dark matter, its nature remains one of the greatest mysteries in physics. Several research directions continue:
-
Next-generation detectors
- Even larger and more sensitive direct detection experiments aim to further penetrate the WIMP parameter space.
- Axion "haloscopes" (e.g., ADMX) and advanced resonant cavity experiments search for axions.
-
Precision cosmology
- Cosmic microwave background (Planck and future missions) and large-scale structure (LSST, DESI, Euclid) observations refine constraints on dark matter density and distribution.
- By combining these data with improved astrophysical models, it is possible to rule out or narrow down non-standard dark matter scenarios (e.g., self-interacting dark matter, warm dark matter).
-
Particle physics and theory
- In the absence of WIMP signals, other alternatives are increasingly considered, such as sub-GeV dark matter, "dark sectors," or even more exotic models.
- Hubble tension – the difference between measured expansion rates of the Universe – has prompted some theorists to explore whether dark matter (or its interactions) could play a role here.
-
Astrophysical research
- Detailed studies of dwarf galaxies, tidal "streams," and stellar motions in the Milky Way halo reveal nuances of small-scale structures that may help distinguish different dark matter models.
Conclusion
Dark matter is an essential part of the cosmological model: it governs the formation of galaxies and clusters and constitutes the majority of the Universe's matter. However, so far we have not been able to directly detect it or fully understand its fundamental properties. From Zwicky's “missing mass” problem to current, highly advanced detectors and observatories – continuous efforts persist to unveil the mysteries of dark matter.
The risk (or scientific value) here is enormous: any final detection or theoretical breakthrough could transform our understanding of particle physics and cosmology. Whether it is a WIMP, axion, sterile neutrino, or a completely unexpected possibility – the discovery of dark matter would become one of the most important achievements of modern science.
Links and further reading
- Zwicky, F. (1933). “Die Rotverschiebung von extragalaktischen Nebeln.” Helvetica Physica Acta, 6, 110–127.
- Rubin, V. C., & Ford, W. K. (1970). “Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions.” The Astrophysical Journal, 159, 379–403.
- Clowe, D., Gonzalez, A., & Markevitch, M. (2004). “Weak-Lensing Mass Reconstruction of the Interacting Cluster 1E 0657–558: Direct Evidence for the Existence of Dark Matter.” The Astrophysical Journal, 604, 596–603.
- Blumenthal, G. R., Faber, S. M., Primack, J. R., & Rees, M. J. (1984). “Formation of Galaxies and Large-Scale Structure with Cold Dark Matter.” Nature, 311, 517–525.
- Tyson, J. A., Kochanski, G. P., & Dell’Antonio, I. P. (1998). “Detailed Mass Map of CL 0024+1654 from Strong Lensing.” The Astrophysical Journal Letters, 498, L107–L110.
- Peccei, R. D., & Quinn, H. R. (1977). “CP Conservation in the Presence of Instantons.” Physical Review Letters, 38, 1440–1443.
Additional sources
- Bertone, G., & Hooper, D. (2018). “A History of Dark Matter.” Reviews of Modern Physics, 90, 045002.
- Tulin, S., & Yu, H.-B. (2018). “Dark Matter Self-Interactions and Small Scale Structure.” Physics Reports, 730, 1–57.
- Peebles, P. J. E. (2017). “Dark Matter.” Proceedings of the National Academy of Sciences, 112, 12246–12248.
Among astronomical observations, particle physics experiments, and innovative theoretical frameworks, scientists are continuously approaching an understanding of the essence of dark matter. It is a journey that changes our perspective on the Universe and may pave the way for new physics discoveries beyond the Standard Model.