A roughly spherical region surrounding a galaxy, containing old stars, globular clusters, and dark matter. The halo extends far beyond the visible disk and contains most of a galaxy's total mass.
From Greek 'galaktikos' (milky, relating to galaxies) + 'halos' (disk of light around the sun or moon). The astronomical usage emerged in the mid-20th century when astronomers realized galaxies had extended, invisible components.
The galactic halo is like a galaxy's invisible crown - it's mostly dark matter that we can't see directly, but it's what keeps the galaxy together! Without this mysterious halo, the stars in galaxy disks would fly apart because there isn't enough visible matter to provide the gravitational glue.
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