Dark Matter Halos & Gravitational Lensing: Structures & Vocabulary

Dark-matter halos dominate the mass budgets of many galaxies and galaxy clusters. Because dark matter emits little or no light, gravitational lensing provides one of the most direct ways to map its projected mass distribution without assuming that mass follows starlight.

Mass curves spacetime and alters light paths. The amount and pattern of distortion depend on total projected mass, including dark matter, stars, and gas. Statistical inversion of many lensed sources reconstructs mass maps.

Galaxy rotation, cluster dynamics, strong-lensing arcs, weak-lensing shear, and collision systems all require more mass than observed baryons provide. Lensing often places most mass where little luminous matter is present.

Strong lensing creates multiple images and arcs; weak lensing measures tiny coherent distortions across many background galaxies. Halo models describe density profiles, concentration, ellipticity, and substructure.