Dark matter is a hypothetical form of matter that is believed to account for approximately 27% of the universe’s mass and energy. It is invisible, intangible, and does not emit, absorb, or reflect light, making it undetectable by conventional means. Dark matter is thought to be a type of matter that does not interact with electromagnetic radiation, such as light, and is only detectable through its gravitational effects on visible matter.Properties of Dark Matter

• Gravitational Interaction: Dark matter interacts with visible matter only through gravity, which is why it is detectable through its gravitational effects on stars and galaxies.
• Invisibility: Dark matter does not emit, absorb, or reflect light, making it invisible to telescopes and other instruments that rely on electromagnetic radiation.
• Massive but Invisible: Dark matter is believed to be massive, but its mass is not detectable through conventional means.
• Widespread Distribution: Dark matter is thought to be distributed throughout the universe, with a higher concentration in the centres of galaxies.

Evidence for Dark Matter

1. Galactic Rotation Curves: The rotation curves of galaxies, which describe how fast stars and gas rotate around the centre of a galaxy, are not consistent with the amount of visible matter present. This suggests that there must be additional mass, such as dark matter, to account for the observed rotation speeds.
2. Gravitational Lensing: The bending of light around massive objects, known as gravitational lensing, has been observed in several cases. This effect is consistent with the presence of dark matter, which is thought to be responsible for gravitational lensing.
3. Large-Scale Structure Formation: The formation of large-scale structures, such as galaxy clusters and superclusters, is influenced by the gravitational effects of dark matter. The observed distribution of these structures is consistent with the presence of dark matter.

Theories and Models

1. Several theories and models have been proposed to explain the nature of dark matter: Weakly Interacting Massive Particles (WIMPs): WIMPs are hypothetical particles that interact with ordinary matter through the weak nuclear force. They are a popular candidate for dark matter.
2. Axions: Axions are hypothetical particles that are predicted by some theories of particle physics. They are believed to be very light and interact weakly with ordinary matter.
3. MACHOs (Massive Compact Halo Objects): MACHOs are hypothetical objects that are massive and compact, such as black holes or neutron stars. They are thought to be present in the halos of galaxies.

Current Research and Challenges

1. Despite significant efforts, the nature of dark matter remains unknown. Current research focuses on Direct Detection: Experiments aim to directly detect dark matter particles interacting with ordinary matter.
2. Indirect Detection: Observations of high-energy particles, such as neutrinos or gamma rays, are used to infer the presence of dark matter.
3. Theoretical Modeling: Theoretical physicists continue to develop and refine models of dark matter.

Dark matter remains one of the most significant unsolved problems in modern physics, and its discovery and understanding could significantly advance our knowledge of the universe and its fundamental constituents.