Properties of dark matter
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Fundamental Physical Properties of Dark Matter
Dark matter is a form of matter that does not absorb, reflect, or interact with electromagnetic radiation, making it invisible to telescopes and undetectable by conventional means. It is neutral, long-lived, and interacts primarily through gravity rather than electromagnetic or strong nuclear forces, which explains why it is so difficult to detect directly 25. Its presence is inferred from gravitational effects on visible matter, such as the rotation curves of galaxies and the dynamics of galaxy clusters 248.
Distribution and Density Profiles in Cosmic Structures
Dark matter is the dominant mass component in galaxy clusters, far outweighing visible matter like stars and gas. Observations show that dark matter is distributed more broadly and smoothly than visible matter, forming extended halos around galaxies and clusters. The density profile of these halos is often described as "cuspy," with higher densities toward the center, although some observations suggest shallower profiles, hinting at possible variations in dark matter properties or alternative models 48. Substructures, or "subhalos," within clusters further support the idea that dark matter is largely collisionless and interacts very weakly with itself and ordinary matter .
Leading Dark Matter Candidates and Particle Properties
The most studied dark matter candidates are Weakly Interacting Massive Particles (WIMPs) and axions. WIMPs are predicted by theories like supersymmetry and are expected to have masses and interaction strengths that allow them to account for the observed dark matter density through standard cosmological processes 23. Axions and other ultralight bosonic particles, sometimes called "wave dark matter," are also strong candidates. These particles can exhibit wave-like behavior on galactic scales, leading to unique density fluctuations and soliton cores in galaxy centers 27.
Other theoretical models propose composite dark matter, such as dark atoms or particles bound by new confining forces, which could have different cosmological and detection signatures . Some models, like geometric dark matter, suggest particles that interact only with fermions and gravity, making them even more elusive .
Interactions and Detection Challenges
Dark matter interacts extremely weakly with ordinary matter, with scattering cross-sections many orders of magnitude below current experimental limits, which explains the lack of direct detection so far 510. Most of our knowledge comes from gravitational effects, but ongoing and future experiments aim to detect dark matter through direct interactions, indirect signals (such as gamma rays or neutrinos from annihilation), or astrophysical observations of its influence on cosmic structures 310.
Role in Cosmic Structure Formation
Dark matter is essential for the formation and evolution of cosmic structures. It provides the gravitational wells in which galaxies and clusters form and grow. Its properties, such as being cold (moving slowly compared to the speed of light) and collisionless, are consistent with the observed large-scale structure of the universe. However, discrepancies at smaller scales have led to the exploration of alternative models, such as warm or self-interacting dark matter 489.
Conclusion
Dark matter is a neutral, non-luminous, and long-lived component of the universe that interacts primarily through gravity. Its distribution shapes galaxies and clusters, and its properties are constrained by both astrophysical observations and particle physics experiments. While WIMPs and axions remain leading candidates, a variety of models are under investigation, and future observations and experiments are expected to further clarify the true nature of dark matter 23456789+1 MORE.
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