The enigmatic blotches are shown up as shadows some 12 billion light years distant. The universe itself is calculated to be 13.8 billion years old. And the oldest known star is 13.2 billion years.

Their existence has been anticipated. But finding a galaxy that emits little or no light is obviously a testing task. Then there’s the added difficulty of proving what it is.

But physicists from ETH Zurich have found six new candidates for what could be immense clouds of primeval gas and rocky debris.

It’s the raw material left over from the dying embers of the Big Bang. Other galaxies have little of this raw ‘stuff’ left. They’re are constantly recycling — and polluting — the stuff that makes them up through the life cycles of their stars.

They’ve published the results of their work in The Astrophysical Journal .

GHOST GALAXIES

Dark galaxies are thought to be rare. For some reason, their make-up makes them ineffective at forming — and igniting — stars.

Instead, they’re filled with vast quantities of gas and matter not hot enough to emit enough light for our telescopes to see.

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Theoretical models of the birth and evolution of our universe suggest they should exist. They could have formed at a time when our universe was too immature for star formation.

But little actual data has been assembled on them.

This is possibly because most of them have been consumed.

Astronomers believe much of their gas and material may have long since been drawn into larger galaxies, fuelling fresh bursts of star formation.

“Despite substantial progress over the past half a century in understanding of how galaxies form, important open questions remain regarding how precisely the diffuse gas known as the intergalactic medium is converted into stars,” the researchers write.

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“One possibility, suggested in recent theoretical models, is that the early phase of galaxy formation involves an epoch when galaxies contain a great amount of gas but are still inefficient at forming stars.”

Finding them, however, is a challenge.

SHADOW HUNT

While dark galaxies are dark, they’re not invisible. So it’s possible for them to distort and cast shadows from light glowing beyond them.

Across the gulf of time and space, quasars — the brightest objects in the universe — are proving capable of casting light on the matter.

A quasar is a bright ball of superheated plasma powered by the supermassive black holes in the centre of most visible galaxies. It’s generated by the immense gravitational forces tearing apart and bashing together matter as it falls towards the event horizon.

“They emit intense UV light, which in turn induces fluorescent emission in hydrogen atoms known as the Lyman-alpha line. As a result, the signal from any dark galaxies in the vicinity of the quasar gets a boost, making them visible,” the researchers write.

So, if a hydrogen-bloated dark galaxy passes close to a quasar, its presence is revealed by distortions within the quasar’s light spectrum.

Several possible dark galaxies were found in 2012. But a recent upgrade to the European Southern Observatory’s Very Large Telescope has enabled astronomers to peer further back in time and space than previously possible.

It’s called the Multi Unit Sectroscopic Explorer (MUSE).

They identified 200 sources of Lyman-alpha light. Deep observations of up to 10 hours for each of six quasar fields eliminated all but six as being likely dark galaxies.

“Every quasar field observed with MUSE will offer the potential to discover new Dark Galaxy candidates and provide crucial information on the early and dark phases of galaxy formation,” the researchers wrote.