Dark matter, the mysterious unseen mass in our universe, plays a crucial role in cosmic structures. While absent in our solar system, its effects are evident in galaxies and galaxy clusters, causing unexpected rotation curves and gravitational lensing.

Scientists use various methods to detect dark matter, including studying galaxy dynamics and cosmic microwave background radiation. This invisible substance makes up about 85% of the universe's matter, shaping our understanding of cosmic evolution and structure formation.

Dark Matter in the Universe

Dark matter in solar system

• Orbits of planets and other objects well-described by Newtonian mechanics and general relativity
• No need for additional unseen mass to explain orbital motions (Mercury, Venus, Earth)
• Precise measurements of spacecraft paths align with predictions based on visible matter
• No significant gravitational discrepancies observed (Voyager 1, New Horizons)

Dark matter evidence in galaxies

• Flat rotation curves
  • Observed orbital velocities of stars and gas in galaxy disks remain constant with increasing distance from center (Milky Way, Andromeda)
  • Inconsistent with expected decrease in velocity based on visible matter distribution
• Gravitational lensing
  • Light from distant galaxies distorted and magnified by intervening matter (Abell 2218 cluster)
  • Strength of lensing effect implies more mass than observed in visible galaxies
  • Weak lensing techniques used to map dark matter distribution on large scales
• Velocity dispersions in elliptical galaxies
  • Stars have higher random velocities than expected from visible mass (M87, NGC 4472)
  • Additional unseen mass needed to explain observed velocity distributions
• Galactic halos extend far beyond visible regions of galaxies, containing significant amounts of dark matter

Galaxy clusters and dark matter

• Virial theorem relates average kinetic energy of galaxies to their average gravitational potential energy
  • Observed galaxy velocities in clusters much higher than expected from visible mass alone (Coma Cluster, Virgo Cluster)
• Hot gas in clusters emits X-rays
  • Temperature and distribution of X-ray emitting gas suggest deep gravitational potential well from unseen mass (Bullet Cluster)
• Galaxy clusters act as powerful gravitational lenses
  • Strength of lensing indicates presence of more mass than observed in visible galaxies and hot gas (Abell 370)

Dark matter vs mass-to-light ratios

• Mass-to-light ratio ($M/L$) is ratio of total mass to total luminosity in a system
  • Higher $M/L$ values indicate presence of more unseen mass relative to visible matter
• Galaxy clusters have $M/L$ ratios significantly higher than individual galaxies (Fornax Cluster, Perseus Cluster)
  • Suggests dark matter dominates total mass budget in these large-scale structures
• Cosmic microwave background (CMB) anisotropies provide snapshot of matter distribution in early universe
  • Analysis of CMB anisotropies indicates dark matter comprises about 85% of total matter content

Dark matter research and theories

• Baryonic matter accounts for only a small fraction of the total matter in the universe
• Cosmological simulations incorporate dark matter to model large-scale structure formation
• Modified gravity theories propose alternatives to dark matter to explain observed gravitational effects
• Particle physics experiments search for potential dark matter candidates beyond the Standard Model