Astronomers spot an unexpected 'ghostly object' in the depths of space

 Scientists have discovered a new type of astronomical object, calling the strange entity a 'window into the dark universe'.

The object known as Cloud-9 is a completely starless, gas-rich cloud of dark matter located 14 million light-years from Earth.

The cloud's core is a vast, compact sphere of neutral hydrogen, about 4,900 light-years across.

That is more than 1,000 times greater than the distance between Earth and the nearest star, Proxima Centauri.

However, despite containing abundant stellar fuel, astronomers have now used the Hubble Space Telescope to confirm that Cloud-9 contains no stars whatsoever.

Scientists say that makes the cloud a building block of a galaxy that never quite formed, left over as a relic from the early universe.

Co-author Dr Andrew Fox, of the European Space Agency and the Space Telescope Science Institute, told the Daily Mail: 'You can think of it as a failed galaxy.

'A ghostly object that didn’t quite have enough mass to become self-gravitating and cross the threshold into star formation.'

Cloud-9 is a previously theoretical type of object known as a Reionization-Limited H I Cloud, or 'RELHIC'.

What makes RELHICs unusual is that they are largely made up of dark matter, the invisible substance which makes up around 26 per cent of the universe's mass.

Although scientists can't directly observe dark matter, including the matter inside Cloud-9, they can tell that something with mass must be there because of the effects of gravity.

'The main piece of evidence for dark matter in this cloud is its size,' says Dr Fox,

'A cloud this size needs a source of gravity to hold it together. There are no stars to provide this gravity, and the neutral hydrogen gas does not contain enough mass, so dark matter must be the culprit. Without it, the cloud would simply fall apart.'

By looking at the radiation emitted by gases in the cloud, scientists estimate that the mass of hydrogen within is about one million times that of the sun.

However, for the cloud not to drift apart, Dr Fox and his colleagues estimate that it must contain around five billion solar masses of dark matter.

This discovery is extremely exciting for astronomers because RELHICs like Cloud-9 offer a snapshot into an exceptionally early moment in the universe's history.

Dr Fox says: 'Theories of galaxy formation predicted that there is a minimum threshold of dark matter required to ignite star formation and turn a dark cloud into a luminous galaxy.

'With Cloud-9, we have an example of an object just below this threshold, containing no stars.'

Although some scientists had thought that RELHICs might exist, they have proven exceptionally hard to find.

If the cloud were much larger, the gases would collapse into stars and form a galaxy; much smaller, and it would have fallen apart and blown away.

Co-author Dr Alejandro Benitez  Llambay, of the Milano-Bicocca University in Milan, told Daily Mail: 'Cloud-9 is a rare "middle ground" survivor.'

'According to our models, fewer than 10 per cent of halos in this mass range remain in such a pristine state, making Cloud-9 a "missing link" in our understanding of how galaxies are born.'

Likewise, since these objects don't contain any stars, RELHICs barely give off any of their own radiation and are exceptionally difficult to detect.

Cloud-9 was first spotted three years ago by the Five-hundred-meter Aperture Spherical Telescope (FAST) in Guizhou, China.

However, it is only now that researchers have been able to use the Hubble Telescope to confirm that it contains no stars, making it very likely to be a RELHIC.

Lead author Dr Gagandeep Anand, of the Space Telescope Science Institute, says: 'Before we used Hubble, you could argue that this is a faint dwarf galaxy that we could not see with ground-based telescopes. They just didn't go deep enough in sensitivity to uncover stars.

'In science, we usually learn more from the failures than from the successes. In this case, seeing no stars is what proves the theory right. It tells us that we have found in the local universe a primordial building block of a galaxy that hasn't formed.'

The discovery of Cloud-9, published in The Astrophysical Journal Letters, also makes it likely that there are more RELHICs out there, even in our local neighbourhood.

China's FAST telescope is particularly good at spotting these kinds of dark gas clouds, so researchers hope to discover more in the future.

Dr Fox adds: 'There absolutely should be more RELHICs out there, and we are looking for more candidates. We need more cases to know whether Cloud-9 is an oddball with unusual properties, or alternatively, is fairly typical.'