Definition of Dark Matter
Dark matter is a form of matter that does not emit, absorb, or reflect light, making it invisible to telescopes and other instruments that detect electromagnetic radiation. In astrophysics, it is inferred to exist through its gravitational effects on visible matter, such as stars and galaxies. Unlike ordinary baryonic matter, dark matter interacts primarily through gravity and possibly the weak nuclear force, but not through electromagnetism.
Key Evidence and Properties
The primary evidence for dark matter comes from observations like galaxy rotation curves, where stars at the edges of galaxies move faster than expected based on visible mass alone, suggesting unseen mass provides extra gravity. Other indicators include gravitational lensing, where light from distant objects bends more than predicted, and the cosmic microwave background, which shows patterns consistent with dark matter's influence during the universe's early formation. Dark matter is estimated to make up approximately 27% of the universe's total mass-energy, compared to 5% for ordinary matter.
Practical Example: Galaxy Clusters
A clear example is the Coma Cluster of galaxies, where the total mass calculated from gravitational effects on member galaxies far exceeds the mass of visible stars and gas. This discrepancy indicates dark matter's presence, holding the cluster together against the outward forces that would otherwise disperse it. Simulations of galaxy formation also rely on dark matter to explain how structures like spiral galaxies form from initial density fluctuations in the early universe.
Importance in Astrophysics
Dark matter plays a crucial role in the structure and evolution of the universe, influencing galaxy formation, large-scale cosmic web development, and the overall expansion dynamics. Understanding it is essential for models of cosmology, such as the Lambda-CDM model, which integrates dark matter with dark energy to describe the universe's composition and fate. Ongoing research, including experiments like those at the Large Hadron Collider, aims to identify dark matter particles to refine these models.