A dwarf galaxy 500 times smaller than our own Milky Way has a giant dark heart, scientists have discovered.
M60-UCD1 is the smallest galaxy known to contain a super-massive black hole at its centre, one with a mass equivalent to 21 million suns.
The finding suggests that huge black holes may be more common than was previously thought.
A black hole is a region where matter has become so densely squeezed that not even light can escape its gravitational pull.
The Milky Way is thought to have a large black hole at its core containing the mass of four million suns.
But the black hole at the heart of M60-UCD1 is five times bigger, accounting for 15% of the small galaxy's total mass.
Scientists believe M60-UCD1 may be the remnant of a larger galaxy that had its outer regions torn away after approaching too close to another monster-sized galaxy, which it now orbits.
A similar story could be behind the formation of other dwarf galaxies, which may also harbour super-massive black holes.
"We don't know of any other way you could make a black hole so big in an object this small," said lead scientist Dr Anil Seth, from the University of Utah, US.
"There are a lot of similar ultra-compact dwarf galaxies, and together they may contain as many super-massive black holes as there are at the centres of normal galaxies."
Astronomers used the Gemini North eight-metre telescope on Mauna Kea, Hawaii, to investigate M60-UCD1.
The dwarf galaxy orbits M60, a colossal galaxy containing a 4.5 billion solar mass black hole. Some time in the future, the two galaxies and their black holes may merge, scientists believe.
M60-UCD1 is about 54 million light years from Earth, but just 22,000 light years from the centre of M60.
Ultra-compact dwarf galaxies are less than a few hundred light years across compared with the Milky Way's 100,000 light years.
The new research is published in the journal Nature.