Scientific estimates show that there should be between about 100 million and 1 billion stellar-mass black holes in the Milky Way. But they aren't easy to locate. They're not like supermassive black holes (SMBH) at the center of galaxies. Those are usually easy to spot for different reasons.
When a SMBH is actively accreting material, the material gathers in an rotating accretion ring, where it heats up and emits energetic radiation. These are called active galactic nuclei (AGN). The brightest of them are called quasars, and they can be seen from vast distances. AGN and quasars emit a pair of powerful astrophysical jets opposite thei axis of rotations, and in some cases these are pointed straight at Earth. We call these blazars, an they announce their presence with a lighthouse-like beam of energy that can't be missed.
Other SMBH are found by their powerful gravity and the effect it has on their surroundings. Sgr A*, the Milky Way's SMBH, is a quiescent black hole that feeds on only a trickle of matter. It emits almost no radiation. It was detected by the way its gravity accelerates the stars that are closest to it, and affects their orbits.
But stellar-mass black holes are much more subdued and more difficult to detect. Still, they must be out there. Modelling predicts them.
Research in The Astrophysical Journal presented an effort to find them in Gaia data. It's titled "Searching for Isolated Black Hole Candidates within 15 pc of the Solar System in Gaia DR3," and the lead author is Abdurakhmon Nosirov. Nosirov is from the Center for Astronomy and Astrophysics, Department of Physics, in Fudan University, Shanghai, China.
"Theoretical models predict that the Galaxy hosts 108–109 black holes formed from the complete gravitational collapse of heavy stars and that most of these black holes are isolated, without any companion," the authors write. If the upper range is 1 billion, and we can't find them, what's stopping us?
"Within 15 pc of the solar system (∼50 lt-yr), there may be a few black holes," the authors write. "If located inside one of the Local Interstellar Clouds—which occupy 5%–20% of this Local Volume—an isolated black hole could produce detectable electromagnetic emission via accretion from the interstellar medium, given the capabilities of current or near-future observatories."
The Local Interstellar Cloud (LIC) is a cloud of partially ionized gas. The Solar System is moving through the LIC, near its edge. Other interstellar clouds, like the G-Cloud, are nearby, and collectively they're referred to as Local Interstellar Clouds (LICs).
This illustration shows the Local Interstellar Cloud (LIC), the Solar System moving through it, and some well-known nearby stars. It also shows the G-Cloud, another nearby interstellar cloud. If there are stellar-mass black holes in the interstellar clouds within 50 light years of the Sun, they should be detectable by their electromagnetic emissions. Image Credit: NASA / Adler / University of Chicago / Wesleyan
But predicting the electromagnetic emissions of black holes within 50 light years is challenging because of "large uncertainties in the expected accretion spectra," according to the authors. The situation is different for black holes within the LICs and those outside of it.
Outside of the LIC, there's very little for black holes to feed on. "Outside these clouds, the accretion rate would be too low; according to our models, the resulting electromagnetic flux is well below the detection thresholds of current and near-future observational facilities," the researchers explain.
Electromagnetic flux is only one way to detect a black hole. The other way is through their gravitational effect on nearby stars, which could be measured with astrometry. Sadly, that won't work in our neighbourhood. "While astrometric detection via gravitational perturbation of nearby stars is conceivable, the local stellar density is too low for this method to be realistically successful," the authors write.
The researchers turned to Gaia and its Data Release 3 to find potential isolated black holes that are accreting from the ISM and found five. (Gaia found six, but one is excluded as a brown dwarf.) While astrometry could be a viable way to confirm them, their location prevents that. "All candidates lie close to the Galactic plane, making them likely spurious astrometric solutions, for instance caused by unmodeled background sources (crowding) and/or unmodeled binarity," the researchers explain.
This figure is a three-dimensional map of the region within 15 parsecs of the Solar System. It shows the locations of the 15 Local Interstellar Clouds, and a coloured x marks the location of each of the six sources. The black star in the center represents the Solar System. Image Credit: Abdurakhmon Nosirov et al 2026 ApJ 1004 21.
So it came down to their modelled infrared and radio emissions.
Gaia didn't gather spectra, so its data is no help. But there are multiple catalogues of emission data covering the 15 parsecs surrounding the Solar System. CatWISE is one of them, a catalogue of infrared sources from NASA's Wide-field Infrared Survey.
RACS, and VLASS are radio catalogues of the sky. RACS is Rapid ASKAP Continuum Survey, and VLASS is the Very Large Array Sky Survey. RACS has observed the radio sky repeatedly with the entire 36-dish ASKAP radio telescope, and VLASS catalogued 10 million individual radio sources.
This image shows some of the 36 radio dishes in ASKAP, the Australian Square Kilometer Array Pathfinder, located in Western Australia. Each dish is 12 m (39 ft) in diameter, and when they're all working together, they form a powerful interferometer. Image Credit: By CSIRO, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=35460691
"Although the detection thresholds of the radio catalogs are close to the fluxes predicted by our models, both models agree that, if these sources were black holes accreting from the interstellar medium, they should be detectable in CatWISE," the researchers write.
Sadly for black hole aficionadoes, no sources were found that matched the Gaia candidates.
"Our search for infrared and radio emission from these sources further suggests that they are unlikely to be black holes accreting from the interstellar medium," the authors write.
This effort wasn't fruitful, but that doesn't mean the search will end. Other research shows that any isolate black hole that was accreting from the ISM would be very faint, and would exhibit a fairly bland spectrum without many features. But that doesn't mean they're undetectable. It just shows that they're likely undectable with observations with single instruments.
"Multiwavelength follow-up campaigns, covering radio, optical, and X-ray bands, are necessary to rule out alternative interpretations and confirm the nature of any candidate," the authors write.
"Future efforts will focus on searching for isolated black holes accreting from the warm, partially ionized interstellar medium in radio and X-ray catalogs," the authors write in their conclusion.
In early 2027, the Square Kilometer Array will be releasing another tranche of data. Will it be the data that allows black hole hunters to find success.
That data "appears especially promising for this endeavor," the researchers conclude.