Dark matter halo

A gravitationally bound, roughly spherical concentration of dark matter surrounding and extending beyond a galaxy or galaxy cluster. Its gravity shapes the motion of stars, gas, and satellite galaxies.

Cold dark matter: A dark matter model in which particles move slowly compared with light during structure formation. Slow-moving particles collapse into halos that grow hierarchically.
Galaxy rotation curve: A graph of orbital speed in a galaxy against distance from its center. Outer rotation speeds reveal mass extending beyond the visible galaxy.
Milky Way: The barred spiral galaxy that contains the Solar System. Stellar motions and satellite orbits constrain the halo surrounding our galaxy.
Modified Newtonian dynamics: A proposed modification of Newtonian dynamics intended to explain galaxy rotation without dark matter. It seeks to explain some halo evidence by changing gravity rather than adding unseen mass.
Core-cusp problem: The tension between cuspy central densities predicted by cold dark matter simulations and some observed galaxy cores. Halo centers may be less dense than simple collisionless models predict.
Virial theorem: A relation connecting the average kinetic and potential energies of a stable, self-gravitating system. It helps describe how a halo's gravity balances its internal motions.
Gravitational lens: A massive object whose gravity bends light from a more distant source. Halo mass can be mapped from the distortion of background light.
Andromeda Galaxy: The nearest large spiral galaxy to the Milky Way. Its satellites and rotation provide an independent view of a massive galaxy halo.
Self-interacting dark matter: A dark matter model in which particles scatter off one another through non-gravitational interactions. Particle collisions can alter central halo density compared with collisionless models.
Missing satellites problem: The discrepancy between predicted numbers of small dark matter halos and observed satellite galaxies. Many low-mass halos may lack visible galaxies, complicating comparisons with observations.

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