What is dark matter?

Dark matter is a form of matter that does not emit, absorb, or reflect light, making it invisible and detectable only through its gravitational effects on visible matter, such as stars and galaxies. It is thought to make up about 27% of the universe's total mass-energy content, while ordinary (baryonic) matter, which includes stars, planets, and gas, constitutes only about 5%. The existence of dark matter was first proposed in the early 20th century to explain discrepancies between the observed rotation curves of galaxies and the predictions made by Newtonian physics and later by general relativity. Observations showed that the outer regions of galaxies rotate at much higher speeds than would be expected if only visible matter were present; this suggests that there is additional, unseen mass exerting gravitational influence. Several lines of evidence support the existence of dark matter, including: 1. **Galactic Rotation Curves**: The rotation speeds of galaxies remain high at large distances from their centers, implying the presence of unseen mass. 2. **Gravitational Lensing**: The bending of light from distant objects by massive foreground objects can be explained by dark matter. The amount of bending indicates more mass than can be accounted for by visible matter alone. 3. **Cosmic Microwave Background (CMB)**: Measurements of the CMB provide insights into the early universe and strongly support the existence of dark matter through its effects on structure formation. 4. **Large-scale Structure**: Observations of the distribution of galaxies and galaxy clusters indicate the existence of a substantial amount of unseen mass necessary to explain the observed structures. Despite extensive research, the exact nature of dark matter remains unknown. It is commonly hypothesized to consist of weakly interacting massive particles (WIMPs) or axions, but no definitive evidence for dark matter particles has yet been observed directly in laboratory experiments.