Cosmology
Dark Matter Halo
The invisible mass surrounding galaxies — 5-10× more than visible matter
A dark matter halo is the gravitationally-bound region of dark matter surrounding each galaxy. Dark matter doesn't emit light but exerts gravity. Halos extend beyond visible galaxy by 10-100×, contain 80-90% of galaxy mass. Provide gravitational glue that explains: (1) flat rotation curves, (2) galaxy formation, (3) gravitational lensing. Dark matter nature unknown — most likely a yet-undiscovered particle (WIMPs, axions). Halos formed during structure formation in early universe via gravitational collapse.
- DM fraction~85% of total mass in galaxy
- Halo extent10-100× visible galaxy
- Milky Way halo mass~10¹² M_sun
- Density profileNFW profile (Navarro-Frenk-White)
- Particle candidatesWIMPs, axions, sterile neutrinos
- Confirmed byRotation curves, lensing, structure formation
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Why DM halos matter
- Galaxy formation. Halos provide gravitational scaffold.
- Structure formation. Drives universe evolution.
- Cosmology. Universe is 27% dark matter.
- Particle physics. Beyond standard model search.
- Galactic dynamics. Rotation curves; flat at large r.
- Cluster dynamics. Bind clusters together.
- Gravitational lensing. Lens strength reveals mass.
Common misconceptions
- Dark matter is dark energy. Different — DM clusters; DE drives expansion.
- DM is a black hole. Distributed; forms halos.
- DM is just dust or ordinary matter we can't see. Different — non-baryonic.
- DM detected by light. Not visible; only gravity.
- All galaxies have same DM. Halos vary in mass and size.
- DM is everywhere uniform. Clumped in halos.
Frequently asked questions
How was dark matter discovered?
Multiple lines of evidence converging. (1) Galactic rotation curves (Vera Rubin 1970s). (2) Galaxy cluster dynamics (Fritz Zwicky 1933). (3) Gravitational lensing (galaxy clusters). (4) CMB power spectrum (WMAP, Planck). (5) Large-scale structure formation. Together: ~85% of mass in universe is dark.
What's the NFW profile?
Navarro-Frenk-White profile (1996). Universal halo density profile from N-body simulations. ρ(r) ∝ 1/[r(r+r_s)²] where r_s is scale radius. Inner profile: ρ ∝ 1/r (cuspy core). Outer: ρ ∝ 1/r³. Most simulated halos fit this. Real halos sometimes show "cores" (rounded centers) — debate about whether NFW correct.
How are halos detected?
(1) Galactic rotation curves — flat at large radii. (2) Stellar streams — past mergers leave evidence. (3) Galaxy clusters — total mass measured via gas X-ray and lensing. (4) Galaxy-galaxy lensing — distortion of background galaxies by foreground halos.
What's the dark matter particle?
Unknown. Candidates: (1) WIMPs (Weakly Interacting Massive Particles) — favored historically; not yet detected. (2) Axions — light bosons; ongoing experiments (ADMX). (3) Sterile neutrinos — heavier neutrinos. (4) Primordial black holes. (5) Modified gravity (MOND) — alternative without dark matter.
Why isn't dark matter visible?
Doesn't interact with electromagnetism. No emission, absorption, or scattering of light. Only gravity. Tests: searches for WIMP scattering off normal matter (XENON, LUX). No detections so far. Nature truly different from ordinary matter.
How do halos form?
Cosmological structure formation. Tiny density fluctuations in early universe collapse first into smallest halos. These merge to form bigger halos. By now: hierarchical structure with halos of all sizes. Galaxy halos contain countless smaller subhalos (substructure). Cluster halos contain galaxy halos within them.
Could halos be modified gravity?
MOND (Modified Newtonian Dynamics) attempts to explain rotation curves without dark matter. Works for individual galaxies. But: galaxy clusters have more "missing mass" than MOND predicts. CMB power spectrum tightly constrains theories. Most cosmologists believe in dark matter.