Dark Matter Halo

A dark matter halo refers to the vast, invisible component of matter that surrounds galaxies and galaxy clusters, providing the gravitational framework within which visible matter such as stars, gas, and dust reside. These halos are critical for understanding galactic formation and dynamics, as they constitute the majority of a galaxy’s total mass and influence its rotation and structure. Despite being undetectable through electromagnetic radiation, their presence is inferred from gravitational effects on visible matter and cosmic background radiation patterns.

Background and Concept

The concept of dark matter halos arose from observations that the rotational velocities of galaxies remained constant at large distances from their centres, contrary to what Newtonian mechanics would predict if only visible matter were present. This discrepancy implied the existence of a massive, unseen component enveloping the galaxy.
Dark matter itself does not emit, absorb, or reflect light, making it undetectable by conventional telescopes. Its presence is revealed primarily through gravitational interactions. A dark matter halo acts as a gravitational scaffold, providing the necessary mass to stabilise galactic rotation and prevent galaxies from dispersing due to their high orbital speeds.

Structure and Distribution

Dark matter halos are generally thought to have a spherical or ellipsoidal shape, extending well beyond the visible boundaries of galaxies. The density of dark matter decreases with increasing distance from the galactic centre, often modelled using profiles such as the Navarro–Frenk–White (NFW) profile or the Einasto profile.
Typical characteristics of dark matter halos include:

• Core Region: A central area of higher density surrounding the galactic bulge.
• Extended Halo: A vast, diffuse region that can extend several hundred thousand light-years from the galactic disc.
• Substructure: Smaller subhalos or clumps, remnants of smaller galaxies or dark matter aggregations absorbed during galactic evolution.

The Milky Way’s dark matter halo is estimated to extend up to 300,000 light-years, containing several times more mass than all its visible stars combined.

Formation and Evolution

Dark matter halos formed early in the universe’s history through the gravitational collapse of density fluctuations present after the Big Bang. Over time, these halos merged hierarchically, leading to larger structures such as galaxies and galaxy clusters.
The ΛCDM (Lambda Cold Dark Matter) cosmological model predicts that cold (slow-moving) dark matter particles clump together under gravity, forming halos that attract baryonic matter — the normal matter that forms stars and planets. The interaction between dark matter and baryonic matter determined the morphology and evolution of galaxies.

Observational Evidence

The existence of dark matter halos is supported by multiple lines of astrophysical evidence:

• Galactic Rotation Curves: The speed of stars orbiting galaxies remains roughly constant with increasing distance from the centre, contradicting the expected decline if only visible mass were present.
• Gravitational Lensing: Light from distant objects is bent more strongly than expected, implying additional unseen mass.
• Cosmic Microwave Background (CMB): Fluctuations in the CMB observed by satellites such as WMAP and Planck provide data consistent with the presence of dark matter.
• Galaxy Cluster Dynamics: The behaviour of galaxies within clusters and the temperature of intracluster gas suggest large amounts of dark matter.

Theoretical Models and Candidates

Several models attempt to explain the nature of dark matter within halos. The most widely supported hypothesis involves cold dark matter (CDM), composed of slow-moving, non-relativistic particles. Possible candidates include:

• WIMPs (Weakly Interacting Massive Particles): Hypothetical particles predicted by supersymmetry theories.
• Axions: Extremely light particles proposed to solve quantum chromodynamics problems.
• Sterile Neutrinos: Neutrinos that do not interact via the weak nuclear force but could contribute to dark matter density.

Alternative theories, such as Modified Newtonian Dynamics (MOND), propose modifications to gravity itself rather than invoking unseen matter, but observational data generally favour the dark matter interpretation.

Role in Galactic Dynamics and Structure Formation

Dark matter halos determine the overall structure and stability of galaxies. They provide the gravitational potential that governs star formation, galactic rotation, and merger processes. During galactic collisions, visible components can be stripped or distorted, yet dark matter halos often pass through each other relatively unaffected, as evidenced by the Bullet Cluster observation — a clear indicator that dark matter interacts gravitationally but not electromagnetically.
The halos also influence galaxy morphology, with massive halos tending to host larger, elliptical galaxies, while smaller halos correspond to spiral or irregular types. Their mass distribution shapes the dynamics of galaxy clusters and the large-scale cosmic web of filaments and voids.

Detection Methods and Challenges

Although dark matter halos cannot be observed directly, several indirect methods have been developed to study them:

• Gravitational Lensing Surveys: Mapping dark matter distributions through the distortion of background galaxies.
• Satellite Galaxy Motions: Analysing the orbital properties of smaller galaxies to infer halo mass.
• N-body Simulations: Computer models replicating cosmic structure formation under dark matter-dominated physics.

Despite extensive research, no direct detection of dark matter particles has yet been achieved. Experiments such as LUX-ZEPLIN, XENONnT, and PandaX continue to search for weakly interacting massive particles using underground detectors shielded from cosmic radiation.

Cosmological Implications

Dark matter halos play an essential role in the large-scale structure of the universe. They act as the fundamental building blocks of the cosmic web, influencing galaxy clustering and the rate of cosmic expansion. Understanding their properties helps constrain cosmological parameters such as the Hubble constant, matter density, and dark energy’s contribution to universal dynamics.