= Solution
The strongest pre-2014 evidence combines large gravitating mass with extreme compactness. In an <X-ray binary>, companion-star radial velocities determine a <binary mass function>; <orbital inclination>, donor properties and distance complete the mass inference. An unseen component above conventional neutron-star <masses> is a black-hole candidate. For example, a 2011 Cygnus X-1 analysis combined optical photometry, spectroscopy and distance information to infer a compact-object mass about $14.8M_\odot$, with model and observational uncertainties.<The Mass of the Black Hole in Cygnus X-1 by Jerome A. Orosz and others>[Abstract and dynamical-model analysis]{ref}
Near <Sagittarius A*>, infrared stellar <astrometry> and radial velocities trace nearly <Keplerian orbits> around a dark compact mass. The concentration of several million solar <masses> in a small volume strongly challenges an extended cluster of ordinary remnants; source confusion, reference-frame errors, distance and possible extended mass must be fitted.<Measuring Distance and Properties of the Milky Way's Central Supermassive Black Hole with Stellar Orbits by Andrea M. Ghez and others>[Abstract and orbital-fit/systematic-error discussion]{ref} Gas and <maser> dynamics provide related compact-mass tests in other nuclei.
<accretion> explains luminous X-rays, rapid variability and broad relativistic disk features. These constrain size and the strong-field environment, but emission alone is not unique to an <event horizon>. Lack of surface-like signatures is supporting, model-dependent evidence. \b[Mass, compactness and consistent <accretion> phenomenology together support the black-hole interpretation]; large-radius mass measurements alone do not directly demonstrate a horizon.
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