The Hunt for Dark Matter

Abdelrahim Bazina • 2025-03-11

Dark Matter remains one of the most enigmatic components of the cosmos. Although it makes up approximately 85% of all matter in the universe, it neither emits or absorbs light, making it invisible and undetectable through direct observational techniques[1],[5],[6]. 𝙄𝙣𝙨𝙞𝙩𝙪𝙩𝙚 - 𝙊𝙪𝙩𝙬𝙤𝙤𝙙 𝘼𝙘𝙖𝙙𝙚𝙢𝙮 𝙍𝙞𝙫𝙚𝙧𝙨𝙞𝙙𝙚

The quest to understand and identify Dark Matter is a monumental challenge in contemporary science, requiring a multidisciplinary approach that combines theoretical physics, astrophysics, and experimental methods.

Evidence for Dark Matter

Dark Matter arises from several key astronomical observations that cannot be explained by just visible matter. One of the primary pieces of evidence comes from galactic rotation curves. According to Newtonian mechanics, the rotational velocity of stars should decrease with distance from the galactic centre

However, observations show that stars at the periphery of galaxies rotate at similar speeds to those near the centre[2]. This unexpected uniformity implies the presence of an unseen mass that exerts additional gravitational force: maintaining the high rotational velocities.

Another significant evidence for Dark Matter is provided by gravitational lensing. This phenomenon occurs when light from distant objects is bent around massive clusters of galaxies. The extent of bending indicates the presence of far more mass than what is visible, implying a substantial amount of unseen Dark Matter[3]. Additionally, the Cosmic Microwave Background (CMB), the residual radiation from the Big Bang, provides a snapshot of the early universe. Variations in the CMB's temperature and density fluctuations suggest the existence of Dark Matter[5], which influenced the formation of large structures in the universe.

Additionally, Dark Matter's role in the evolution of the universe is crucial.

Its presence is indispensable to the formation of galaxies and galaxy clusters. Dark Matter's gravitational effects allowed for the clumping of matter in the early universe, countering the radiation’s dispersive effects. This clumping was essential for the formation of large-scale structures such as the Milky Way and the development of stars, planets, and ultimately life. Despite its invisibility, Dark Matter has been critical to the evolution of our universe and to the emergence of stars, planets, and even life. This is because Dark Matter carries five times heavier than ordinary matter and does not directly interact with light[7]. Both these properties were critical to the creation of structures such as galaxies within the relatively short time span we know to be a typical galaxy lifetime.

The leading theory, called ‘cold dark matter’, reproduces simulations of large-scale structures in the universe well. However, it struggles at smaller scales, such as the cores of low-mass galaxies. This has led to other proposals such as ‘warm dark matter’, a lighter and faster particle[5].

Philip Mocz at Princeton and colleagues at Princeton found that these models lead to distinct distributions of dark matter in the universe, and, subsequently, a different galaxy formation[5]. Both of these candidates, however, coalesce into a web of filaments millions of light years long. This variation in filament structures could refine our understanding of galaxy formation and guide future observational efforts, including those of the James Webb telescope.

Properties of Dark Matter

From the evidence at hand, scientists have deduced several properties of Dark Matter. It must interact predominantly through gravity, given its significant influence on galactic rotation and gravitational lensing. However, it does not emit, absorb, or reflect light, which accounts for its invisibility[8]. Furthermore, Dark Matter appears to be cold, meaning it moves slowly compared to the speed of light, and it clusters around galaxies and galaxy clusters, shaping the universe's large-scale structure[1].

Matter is not necessarily composed of atoms; it is, rather, anything which interacts with gravity. Most of it can be made of something entirely distinct. There is no reason that matter must always consist of charged particles. But matter that has no electromagnetic interactions will be invisible to our eyes. So-called Dark Matter carries no (or as yet undetectably little) electromagnetic charge[7]. Hypotheses on the Nature of Dark Matter

Several hypotheses have been proposed to explain the nature of Dark Matter. One leading candidate is Weakly Interacting Massive Particles (WIMPs). These hypothetical particles interact via the weak nuclear force and gravity but not via electromagnetic force[9], making them invisible and difficult to detect. WIMPs are appealing because they naturally arise in many extensions of the Standard Model of particle physics, such as supersymmetry. Another candidate is axions, extremely light particles that could solve certain theoretical issues in particle physics and also act as Dark Matter. Axions were initially proposed to solve the strong CP problem in quantum chromodynamics, and they could potentially be detected via their conversion to photons in the presence of a strong magnetic field. Sterile neutrinos, a type of neutrino that does not interact via the standard weak force, are also considered potential Dark Matter candidates. These particles are interesting because they could help explain the small masses of active neutrinos observed in neutrino oscillation experiments. Methods of Searching for Dark Matter

The search for Dark Matter is conducted through various methods, broadly categorised into direct detection, indirect detection, and collider experiments. Direct detection experiments aim to observe Dark Matter particles as they interact with normal matter. These experiments, such as those conducted in deep underground laboratories, attempt to detect the rare collisions between Dark Matter particles and atomic nuclei. Experiments like XENON1T and LUX-ZEPLIN use large volumes of liquid xenon to search for tiny flashes of light and ionisation signals caused by these interactions.

Indirect detection focuses on observing the byproducts of Dark Matter interactions. For instance, if WIMPs annihilate each other, they should produce gamma rays, neutrinos, or other particles detectable by telescopes. Instruments like the Fermi Gamma-ray Space Telescope and the IceCube Neutrino Observatory are used to search for these signals. The detection of an excess of gamma rays from the center of our galaxy, where Dark Matter is expected to be densest, could provide a clue to its nature.

Collider experiments, like those at the Large Hadron Collider (LHC), attempt to produce Dark Matter particles by recreating high-energy conditions similar to those of the early universe. By smashing protons together at near-light speeds, physicists hope to produce Dark Matter particles that can be inferred from missing energy and momentum in the particle collision. The detection of such missing energy events could signal the production of Dark Matter particles. Current Research Trends

The study of Dark Matter is at the forefront of modern astrophysics and particle physics. Recent research has focused on several intriguing aspects, including the role of Dark Matter in the formation of supermassive black holes. It is hypothesised that Dark Matter could provide the additional mass necessary for the rapid formation of these massive entities in the early universe. Observations of high-redshift quasars, which are believed to be powered by supermassive black holes, suggest that these black holes formed much earlier than previously thought. This rapid formation could be facilitated by the presence of Dark Matter, which would accelerate the collapse of gas clouds and the growth of black holes.

In addition to exploring the connection between Dark Matter and black holes, researchers are investigating alternative theories to Dark Matter, such as Modified Newtonian Dynamics (MOND). MOND proposes that the laws of gravity themselves might be different on cosmic scales, eliminating the need for Dark Matter. According to MOND, the gravitational force experienced by objects in a galaxy is stronger than predicted by Newtonian mechanics, thus explaining the flat rotation curves of galaxies without invoking unseen mass. While MOND has had some success in explaining galactic phenomena, it struggles to account for observations on larger scales, such as galaxy clusters and the CMB. Nonetheless, it remains an active area of research, with scientists continually testing its predictions against new data.

Current Status and Future Prospects

Despite extensive efforts, Dark Matter has not yet been directly detected. Experiments like LUX-ZEPLIN and XENON1T have set stringent limits on WIMP interactions, narrowing down the possible characteristics of these particles. Meanwhile, astrophysical observations continue to provide indirect evidence supporting the existence of Dark Matter. Theoretical advancements and technological improvements offer hope for future breakthroughs. The discovery of Dark Matter would not only solve one of the most significant mysteries in modern science but also potentially open up new avenues in physics. For instance, understanding Dark Matter could shed light on the unification of forces, the nature of quantum gravity, and the early universe's conditions.

Conclusion

The hunt for Dark Matter is a testament to human curiosity and the relentless pursuit of understanding the universe. While the nature of Dark Matter remains elusive, the convergence of evidence from galactic rotation curves, gravitational lensing, and the CMB strongly supports its existence. The ongoing search through direct detection, indirect detection, and collider experiments underscores the innovative and interdisciplinary approach scientists are taking. Whether or not Dark Matter will be detected in the near future remains uncertain, but the quest itself continues to push the boundaries of our knowledge and technological capabilities. As we develop more sensitive instruments and refine our theoretical models, we move closer to uncovering the true nature of this mysterious component of the universe.