Overview

Dark matter is a form of matter that does not interact with electromagnetic radiation, rendering it invisible to current detection methods. Its presence is inferred from gravitational effects on visible matter, such as the rotational speeds of galaxies and gravitational lensing. Dark matter is estimated to constitute approximately 27% of the universe's total mass-energy content, with dark energy accounting for about 68%, and ordinary (baryonic) matter making up the remaining 5%.

Historical Background

The concept of dark matter emerged in the early 20th century. In 1933, Swiss astronomer Fritz Zwicky observed that the Coma galaxy cluster's galaxies moved faster than could be explained by visible mass alone, suggesting the presence of unseen mass. In the 1970s, Vera Rubin and Kent Ford provided further evidence by studying spiral galaxies' rotation curves, which remained flat at increasing distances from the galactic center, indicating additional unseen mass.

Observational Evidence

Galaxy Rotation Curves

In spiral galaxies, stars orbit the galactic center. If only visible matter were present, stars farther from the center would orbit more slowly, similar to planets in the Solar System. However, observations show that stars maintain constant orbital speeds regardless of their distance from the center, implying the existence of a dark matter halo surrounding the galaxy.

Gravitational Lensing

Gravitational lensing occurs when massive objects, like galaxy clusters, bend the light from background objects. The degree of lensing observed often exceeds what visible matter can account for, suggesting additional mass from dark matter. For instance, the Bullet Cluster provides evidence for dark matter through the separation of visible matter and gravitational mass inferred from lensing.

Cosmic Microwave Background (CMB)

The CMB is the afterglow of the Big Bang, providing a snapshot of the early universe. Anisotropies in the CMB reveal information about the universe's composition. Data from missions like Planck indicate that dark matter constitutes a significant portion of the universe's total mass-energy density.

Theoretical Models

Cold Dark Matter (CDM)

CDM posits that dark matter particles move slowly compared to the speed of light and interact weakly with ordinary matter and radiation. This model successfully explains large-scale structures in the universe and the formation of galaxies.

Warm and Hot Dark Matter

Warm dark matter particles have higher velocities than CDM particles but are still non-relativistic. Hot dark matter particles move at relativistic speeds. These models have different implications for structure formation, with hot dark matter leading to a smoother universe and less small-scale structure.

Candidates for Dark Matter

Weakly Interacting Massive Particles (WIMPs)

WIMPs are hypothetical particles that interact via the weak nuclear force and gravity. They are a leading candidate for dark matter and are the focus of various detection experiments.

Axions

Axions are hypothetical particles proposed to solve the strong CP problem in quantum chromodynamics. They are light and interact weakly with matter, making them a candidate for dark matter.

Sterile Neutrinos

Sterile neutrinos are hypothetical neutrinos that do not engage in weak nuclear interactions, unlike regular neutrinos. They are considered a potential component of dark matter.

Detection Efforts

Direct Detection

Direct detection experiments aim to observe dark matter particles interacting with ordinary matter. These experiments are typically conducted deep underground to shield from cosmic rays. Examples include the XENON and LUX experiments.

Indirect Detection

Indirect detection involves observing the byproducts of dark matter annihilation or decay, such as gamma rays, neutrinos, or positrons. Instruments like the Fermi Gamma-ray Space Telescope search for these signals.

Collider Experiments

Particle accelerators like the Large Hadron Collider (LHC) attempt to produce dark matter particles through high-energy collisions. While no direct evidence has been found, these experiments constrain possible properties of dark matter candidates.

Recent Developments

In 2023, the European Space Agency launched the Euclid space telescope to map billions of galaxies and investigate dark matter and dark energy. Euclid aims to create a detailed atlas of the cosmos, enhancing our understanding of the universe's structure and the role of dark matter.

Challenges and Alternative Theories

Despite extensive research, the exact nature of dark matter remains unknown. Some scientists propose modifying gravity theories, such as Modified Newtonian Dynamics (MOND), to explain observations without invoking dark matter. However, these theories face challenges in explaining all observed phenomena attributed to dark matter.

Conclusion

Dark matter is a fundamental component of the universe, influencing its structure and evolution. While its exact nature remains elusive, ongoing research through observational astronomy, particle physics, and cosmology continues to shed light on this mysterious substance.