Evidence for existence of dark matter
Evidence for dark matter comes from multiple, independent lines of observation across astrophysics and cosmology. Here are the main ones:
1. Galaxy Rotation Curves
- Observation: Spiral galaxies rotate with nearly constant speed at large distances from the center.
- Problem: Based on visible matter alone, outer stars should orbit much more slowly.
- Inference: There must be additional unseen mass—dark matter—exerting gravitational influence.
2. Gravitational Lensing
- Observation: Light from background objects is bent by massive foreground objects (gravitational lensing).
- Example: The Bullet Cluster (two colliding galaxy clusters) shows separation between the visible mass (hot gas) and the mass inferred from lensing.
- Inference: Most mass doesn’t interact electromagnetically—consistent with dark matter.
3. Cosmic Microwave Background (CMB)
- Observation: Precision measurements from Planck and WMAP satellites show patterns in the CMB.
- Model: To match observed temperature fluctuations, cosmologists must include about 27% dark matter in the total energy density.
- Inference: Dark matter is needed to explain structure formation in the early universe.
4. Structure Formation
- Observation: Galaxies and large-scale structures formed relatively quickly after the Big Bang.
- Problem: Visible matter alone wouldn’t clump fast enough due to radiation pressure in the early universe.
- Inference: Dark matter, being non-interacting with radiation, allowed early structure growth.
5. Galaxy Cluster Dynamics (Zwicky’s Observations)
- Observation: In the 1930s, Fritz Zwicky studied the Coma cluster and found galaxies were moving too fast to be held together by visible mass.
- Term: He coined “dunkle Materie” (dark matter).
- Inference: Extra unseen mass must be present.
6. Observations of Dwarf Galaxies
- Observation: Dwarf galaxies exhibit high mass-to-light ratios.
- Inference: These galaxies must contain far more mass than is visible, consistent with dark matter dominance.
7. Big Bang Nucleosynthesis (BBN)
- Observation: The observed abundances of light elements (like H, He, and Li) match predictions from BBN only if most of the matter is non-baryonic.
- Inference: Ordinary matter (protons, neutrons) is not enough—supporting the existence of dark matter.
Summary Table
| Evidence Type | Main Message |
|---|---|
| Galaxy rotation curves | Dark matter needed to explain flat velocity profiles |
| Gravitational lensing | More gravity than visible mass indicates |
| CMB | Dark matter needed for observed temperature anisotropies |
| Structure formation | Early clumping implies non-interacting mass |
| Cluster dynamics | Galaxies move as if much more mass is present |
| Dwarf galaxies | High mass-to-light ratios |
| Nucleosynthesis | Element abundances require extra (non-baryonic) matter |