Dark Matter
Dark matter is an unseen form of matter that does not emit light but reveals itself through gravity, making up most of the matter in the universe and shaping galaxies and cosmic structure.
Definition
Dark matter is a non-luminous, non-baryonic form of matter that interacts primarily through gravity, inferred from its gravitational effects on visible matter and light, and required to explain the dynamics of galaxies and clusters and the formation of cosmic structure.
Scope
This area covers the gravitational evidence for dark matter from galaxy rotation curves, galaxy clusters, gravitational lensing, and the cosmic microwave background, the leading particle candidates and theoretical models for its identity, and the experimental efforts to detect it directly, indirectly, or through production at colliders.
Sub-topics
Core questions
- What is the evidence that dark matter exists?
- What could dark matter be made of?
- How might dark matter be detected?
Key concepts
- Cold dark matter
- Rotation curves
- Gravitational lensing
- Galaxy clusters
- Relic abundance
- WIMPs
- Non-baryonic matter
Key theories
- Cold dark matter
- Most matter in the universe is non-baryonic and moves slowly, or cold, allowing it to clump gravitationally and seed the formation of galaxies and large-scale structure as in the standard Lambda-CDM model.
- Dynamical mass discrepancy
- The orbital speeds of stars and galaxies imply far more gravitating mass than the visible matter can account for, the original and still central evidence for dark matter.
Clinical relevance
Dark matter is essential to the standard cosmological model: it constitutes about a quarter of the cosmic energy budget, provides the gravitational scaffolding on which galaxies and clusters form, and is required to fit the cosmic microwave background, galaxy rotation curves, and large-scale structure simultaneously.
History
Zwicky inferred missing mass in galaxy clusters in 1933, and Rubin and Ford's flat galaxy rotation curves in the 1970s made the case compelling; later evidence from lensing, the bullet cluster, and the cosmic microwave background established dark matter as a cornerstone of cosmology, though its particle identity remains unknown.
Debates
- Particle dark matter versus modified gravity
- While most evidence favors a new form of matter, some propose that modifications of gravity could mimic dark matter on galactic scales; the success of cold dark matter on cosmological scales and observations like the bullet cluster strongly favor the particle interpretation but the debate persists.
Key figures
- Fritz Zwicky
- Vera Rubin
- Jeremiah Ostriker
- James Peebles
Related topics
Seminal works
- zwicky1933
- rubin1970
Frequently asked questions
- How do we know dark matter exists if we cannot see it?
- Its gravity is detectable: it bends light, holds fast-moving galaxies in clusters, flattens galaxy rotation curves, and shapes the cosmic microwave background, all pointing consistently to far more mass than the visible matter provides.
- Could dark matter just be ordinary matter that is hard to see?
- Faint ordinary matter exists, but it cannot account for the total: Big Bang nucleosynthesis and the cosmic microwave background limit baryons to a few percent of the energy budget, so most dark matter must be a non-baryonic substance.