A black hole that could be four times the size of the Sun has been spotted "floating free" through our galaxy on its own.
Researchers at the University of California said there could be up to 200 million such free-floating black holes in our Milky Way.
Black holes are invisible unless part of a stellar binary or surrounded by an accretion disk.
Scientists used gravitational micro-lensing, where the black hole distorts and brightens light coming from 'behind' it as seen from Earth.
The researchers are still unsure whether the object is a black hole or a neutron star (the heavy core left behind after a supernova explosion).
The team, led by graduate student Casey Lam and Jessica Lu, a UC Berkeley associate professor of astronomy, estimated that the mass of the invisible compact object was between 1.6 and 4.4 times that of the Sun.
As astronomers think that the leftover remnant of a dead star must be heavier than 2.2 solar masses in order to collapse to a black hole, the researchers caution that the object could be a neutron star instead of a black hole.
Neutron stars are also dense, highly compact objects, but their gravity is balanced by internal neutron pressure, which prevents further collapse to a black hole.
Whether a black hole or a neutron star, the object is the first dark stellar remnant – a stellar 'ghost' – discovered wandering through the galaxy unpaired with another star.
Determining how many of these compact objects populate the Milky Way will help astronomers understand the evolution of stars – in particular, how they die – and of our galaxy.
It could also reveal whether any of the unseen black holes are primordial black holes, which some cosmologists think were produced in large quantities during the Big Bang.
The analysis by Lam, Lu and their international team has been accepted for publication in The Astrophysical Journal Letters.
The team also concluded that the likely population of black holes in the galaxy is 200 million – about what most theorists predicted.
The length of the lensing event was the main tip-off that the researchers were looking at a black hole, Lam said. In 2020, she showed that the best way to search for black hole micro-lenses was to look for very long events.
Only 1% of detectable micro-lensing events are likely to be from black holes, she said, so looking at all events would be like searching for a needle in a haystack.
But, Lam calculated, about 40% of micro-lensing events that last more than 120 days are likely to be black holes.
"How long the brightening event lasts is a hint of how massive the foreground lens bending the light of the background star is," she said.
"Long events are more likely due to black holes. It's not a guarantee, though, because the duration of the brightening episode not only depends on how massive the foreground lens is, but also on how fast the foreground lens and background star are moving relative to each other.
"However, by also getting measurements of the apparent position of the background star, we can confirm whether the foreground lens really is a black hole."
Watch: Footage shows black hole within Milky Way