Dark matter is one of the most enigmatic components of the universe, constituting approximately 27% of its total mass-energy content. Unlike ordinary matter, which makes up roughly 5% of the universe and includes stars, planets, and galaxies, dark matter is invisible and does not interact with electromagnetic forces. This lack of interaction with light or other forms of electromagnetic radiation renders dark matter detectable only through its gravitational influence.
Dark matter does not emit, absorb, or reflect light or any other form of electromagnetic radiation. This property makes it invisible to telescopes and other instruments that rely on electromagnetic signals for detection. As a result, dark matter cannot be directly observed, and its presence is inferred through its gravitational effects on visible matter and the curvature of spacetime.
Despite its invisibility, dark matter exerts substantial gravitational forces that influence the motion of galaxies, galaxy clusters, and the large-scale structure of the universe. Its gravitational pull is essential for maintaining the rotational speeds of galaxies and preventing them from disintegrating. Additionally, dark matter contributes to gravitational lensing, where it bends the path of light from distant objects, providing further evidence of its existence.
Dark matter is significantly more abundant than ordinary (baryonic) matter. It accounts for about 85% of the total matter in the universe, while ordinary matter comprises less than 15%. Dark matter is distributed in a way that forms an intricate cosmic web, acting as the scaffolding for galaxies and larger structures to form and evolve.
Unlike ordinary matter, which is composed of protons, neutrons, and electrons, dark matter is believed to consist of exotic, non-baryonic particles. These particles do not interact via the electromagnetic force, making them elusive and challenging to detect directly with current technology.
Observations of the rotational speeds of stars within galaxies reveal that stars at the outer edges orbit at roughly the same speed as those closer to the center. According to Newtonian mechanics and the distribution of visible matter, stars farther from the galactic center should orbit more slowly. The discrepancy suggests the presence of additional, unseen mass—dark matter—to provide the necessary gravitational pull.
Gravitational lensing occurs when the light from distant galaxies and stars is bent by the gravitational field of massive objects, such as galaxy clusters, situated between the light source and the observer. The degree of light bending observed is greater than what can be accounted for by visible matter alone, indicating the presence of additional mass in the form of dark matter.
The CMB is the afterglow of the Big Bang, providing a snapshot of the early universe. Detailed measurements of the CMB's temperature fluctuations and their distribution support the existence of dark matter. These fluctuations are consistent with models that include dark matter, influencing the density and distribution of matter in the universe's infancy.
The formation and distribution of galaxies and galaxy clusters on cosmic scales align with predictions that include dark matter. Dark matter's gravitational influence facilitates the clumping of ordinary matter, serving as the framework upon which galaxies and larger structures can form and evolve over billions of years.
WIMPs are among the leading candidates for dark matter. These hypothetical particles interact primarily through gravity and possibly the weak nuclear force, but not electromagnetism. Their mass and interaction properties make them a significant focus of both theoretical and experimental research.
Axions are ultra-light particles proposed as a solution to certain problems in particle physics, such as the strong CP problem. They possess very low mass and weak interactions, making them suitable candidates for dark matter. Axion detectors are currently being developed and tested in various experiments.
Sterile neutrinos are a type of neutrino that does not interact via the weak nuclear force, unlike the known active neutrinos. They interact only through gravity, making them another viable candidate for dark matter. The existence of sterile neutrinos could also have implications for our understanding of neutrino masses and oscillations.
Beyond WIMPs, axions, and sterile neutrinos, other theoretical particles such as supersymmetric particles and particles from hidden sector theories have been proposed as dark matter candidates. These candidates often emerge from extensions to the Standard Model of particle physics and remain subjects of active research.
It's crucial to differentiate dark matter from dark energy, as they are distinct components of the universe with different properties and effects. While dark matter accounts for approximately 27% of the universe's mass-energy content and plays a role in the gravitational binding of structures, dark energy constitutes about 68% and is responsible for the accelerated expansion of the universe. Together, they dominate the cosmos, leaving ordinary matter as a minor component.
The concept of dark matter originated in the 1930s when Swiss astronomer Fritz Zwicky observed the motion of galaxies within the Coma Cluster. He found that the visible mass was insufficient to account for the observed gravitational effects, leading him to propose the existence of unseen mass—dark matter.
In the 1970s, astronomer Vera Rubin conducted extensive studies of galaxy rotation curves. Her observations confirmed that the rotational speeds of stars in spiral galaxies did not decrease with distance from the galactic center as expected based solely on visible matter. Rubin's work provided compelling evidence for the existence of dark matter.
Numerous experiments and observational projects are actively seeking to detect dark matter particles directly or indirectly. These include:
Dark matter is fundamental in the formation and stability of galaxies. Its gravitational pull provides the necessary framework to accumulate ordinary matter, leading to the formation of stars and galactic structures. Without dark matter, galaxies would lack the mass required to hold together, especially given their rapid rotation speeds.
On a cosmological scale, dark matter influences the large-scale structure of the universe. It facilitates the clustering of matter, leading to the formation of galaxy clusters and cosmic filaments. The distribution and density of dark matter shapes the overall geometry and evolution of the universe.
The CMB provides a window into the early universe, with its temperature fluctuations revealing information about the distribution of matter at that time. The presence of dark matter affects these fluctuations, and precise measurements of the CMB align with models that include dark matter, reinforcing its necessity in cosmological theories.
Despite extensive efforts, dark matter has yet to be detected directly. Its weak interaction with ordinary matter makes it exceptionally challenging to observe, necessitating the development of increasingly sensitive detection methods and experiments.
While several candidates have been proposed, the true nature of dark matter remains unknown. Theories continue to evolve, and new models are being developed to explain dark matter's properties and interactions, often extending beyond the Standard Model of particle physics.
Understanding dark matter is not only a challenge for astrophysics but also for particle physics. Discovering dark matter particles would have profound implications for both fields, potentially leading to new physics beyond current theories and models.
Dark matter is a cornerstone of modern cosmology, crucial for explaining the formation, structure, and dynamics of the universe. Despite being invisible and elusive, its gravitational influence is undeniable, shaping galaxies and cosmic structures across vast distances. Ongoing research and advancements in detection technologies continue to bring scientists closer to unraveling the mysteries of dark matter. Solving this cosmic puzzle could revolutionize our understanding of the universe, bridging gaps between astrophysics and particle physics.