A compact astronomical object proposed as a constituent of a galactic dark matter halo. It can be sought through gravitational microlensing even if its emitted light is undetectable. Stellar objects and remnants consume the cosmological baryon budget; primordial black holes do not obey that particular formation-budget argument. Exclusion bounds depend on the object's mass, the halo model and survey selection.
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A Massive Compact Halo Object (MACHO) is a hypothetical type of astronomical object that is proposed to exist in the halo of galaxies, particularly the Milky Way. MACHOs are believed to be composed of ordinary matter, mainly in the form of objects such as brown dwarfs, white dwarfs, neutron stars, or even black holes. They are "compact" because they have a relatively small size compared to their mass, leading to a high density.