No object except for a black hole can have an event horizon, so evidence for its existence offers resounding proof of black holes in space.
Gravity draws gas from a companion star onto a black hole in a swirling pattern.
As the gas nears the event horizon, a strong gravitational red shift makes it appear redder and dimmer. By comparing the energy output of different types of X-ray novae while they were inactive, the Chandra team determined that systems suspected of harboring black holes emitted only one percent as much energy as systems with neutron stars. It's a bit odd to say we've discovered something by seeing almost nothing, but, in essence, this is what we have done. If a collapsed star is a neutron star with a solid surface, energy must be released when infalling material strikes that surface. In contrast, if the accreting object is a black hole, only a small amount of energy can escape before it crosses the event horizon and vanishes forever.