Dark matter is a hypothetical form of matter that does not interact with light or electromagnetic radiation, making it invisible and difficult to detect directly. It is called "dark" because it does not emit, absorb, or reflect light, hence making it essentially transparent and invisible to traditional methods of observation. Dark matter is inferred by its gravitational effects on visible matter and structures in the universe.
The existence of dark matter is proposed to explain various astronomical observations that cannot be accounted for by the known visible matter (such as stars, galaxies, and gas clouds) and the effects of gravity from visible objects alone. Some of the evidence for dark matter includes:
Galactic Rotation Curves: Observations of the rotational speeds of galaxies indicate that they rotate much faster than can be explained by the visible matter within them. Dark matter is postulated to provide the additional gravitational pull necessary to explain these high speeds.
Galaxy Cluster Dynamics: The motion of galaxies within galaxy clusters suggests the presence of significantly more mass than is accounted for by visible matter. Dark matter is believed to constitute a significant portion of the total mass in these clusters, providing the gravitational glue that holds them together.
Gravitational Lensing: The bending of light by the gravitational pull of massive objects can be used to indirectly detect dark matter. Observations of gravitational lensing indicate the presence of additional mass in galaxy clusters that cannot be attributed to visible matter.
The presence of dark matter has far-reaching effects on the universe. It plays a critical role in the formation and evolution of cosmic structures, such as galaxies and galaxy clusters, by providing the gravitational scaffolding necessary for the clumping of visible matter. Without dark matter, the observed structures in the universe would not have been able to form and evolve as we observe them.
Dark matter also affects the large-scale structure of the cosmos. Its gravitational pull slows down the expansion of the universe, counteracting the repulsive effect of dark energy. Dark matter provides the "cosmic glue" that helps to organize galaxies and clusters into vast cosmic filaments and web-like structures.
Despite its significant impact on the universe, the true nature of dark matter remains a mystery. Various theories propose that dark matter could consist of new types of particles that interact weakly with ordinary matter. Numerous experiments are underway to directly detect dark matter particles or to produce them in high-energy particle accelerators. However, to date, the direct detection of dark matter remains elusive, and its precise composition and properties remain one of the outstanding puzzles in modern astrophysics and particle physics.