Astronomers using the Hubble orbital telescope found a "supermassive" black hole lurking at the heart of the only dwarf galaxy ever observed to host one.

The surprise was discovered at the core of a tiny but incredibly dense galaxy, M60-UCD1, about 50 million light years from Earth, the team reported in the journal Nature.

M60-UCD1 is packed with some 140 million stars but is only 300 light years across - 1/500th of the diameter of the Milky Way.

Dr Anil Seth from the Physics and Astronomy University of Utah, who lead the team of astronomers, said black holes are enigmatic phenomena whose gravitational force is so extreme that not even light can escape their clutch.

"A black hole has mass but it has no volume," Dr Seth told 774 ABC Melbourne.

"All the mass is compressed down to a point, so what makes a black hole is that light can't escape.

"So, if you don't launch yourself fast enough from the surface of the Earth you can't leave the surface of the Earth.

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"In a black hole, even if you launch yourself at the speed of light, which is the fastest you can go, you can't get out of a black hole."

Dr Seth said supermassive versions of black holes have until now only been seen at the centre of large galaxies, including our own, but never one so small.

"The milky way has a black hole that's about four million times the mass of the sun, the black hole we found in this object is actually five times bigger than the black hole in the milky way," Dr Seth said.

"But the galaxy is about 1,000 times less massive, it's a really tiny little galaxy, so it's actually the smallest and the least massive galaxy we know that has a black hole."

The astronomers were especially taken aback when they calculated that the hole accounts for a whopping 15 per cent of the galactic mass.

The hole has the mass of 20 million suns, making it five times heavier than the black hole at the centre of the Milky Way.

One explanation could be that M60-UCD1 was once part of a much larger galaxy, which split and left one section with the black hole.

If this theory is wrong and further sky-gazing shows ultra-compact dwarf galaxies do typically harbour a supermassive black hole, it may be time for a rethink.

It would mean there could be twice as many black holes in our region of the universe than previously estimated, the journal said.

ABC/AFP