Black holes, by their nature, are invisible unless part of a stellar binary or surrounded by an accretion disk. Most stellar-sized black holes aren’t, but astronomers have been searching for them through gravitational microlensing events, where the black hole brightens and distorts light from stars toward the galactic center. Astronomers may have found the first free-floating black hole, though more data is needed to rule out a neutron star.
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Source: ScienceDaily