Dark matter is a hypothetical form of matter that does not interact with light or other forms of electromagnetic radiation. Its existence is inferred from its gravitational effects on visible matter and the large-scale structure of the universe. Dark matter is estimated to make up about 85% of the total matter in the universe, while ordinary matter (atoms) accounts for only about 15%.
The name "dark matter" refers to the fact that it does not emit, absorb, or reflect electromagnetic radiation, making it invisible to direct observation. Its presence is inferred through its gravitational influence on visible matter and the way it affects the motion of galaxies and galaxy clusters.
The exact nature of dark matter remains unknown, but several theories and candidates have been proposed. One of the leading candidates for dark matter is a type of particle called a WIMP (Weakly Interacting Massive Particle). WIMPs are hypothetical particles that interact weakly with normal matter and other particles, making them difficult to detect directly.
Axions, on the other hand, are another class of hypothetical particles that have been proposed as a solution to some problems in particle physics, including the strong CP problem. The strong CP problem is related to the behavior of the strong nuclear force, which is responsible for holding atomic nuclei together.
Axions were originally proposed to explain why certain interactions involving the strong nuclear force violate the conservation of a symmetry called CP (charge conjugation and parity). Axions are hypothetical particles with very low mass and low interaction strengths with normal matter. They are postulated to interact extremely weakly with ordinary matter and have properties that could make them a viable candidate for dark matter.
Axions are actively being studied and searched for in experiments, particularly in the field of astroparticle physics. If axions exist and are identified, they could help explain some unsolved mysteries in particle physics and potentially contribute to our understanding of dark matter.
It's important to note that despite extensive efforts, dark matter and axions have not yet been directly detected. Researchers are continuing to explore various detection techniques and conduct experiments to uncover the true nature of dark matter and investigate the existence of axions.