An enormous string of galaxies 300 million light-years long has been discovered in the remote Universe, challenging existing theories about how the Universe evolved.

The remote area was formed very early, at a time when the Universe was a fifth of its present age and the presence of the galaxy string defies existing models, which can not explain how a string this big could have formed so long ago.

This is the first time astronomers have been able to map an area in the early Universe big enough to reveal such a galaxy structure.

ANU astronomer Dr Paul Francis, who coordinated the international research team, said the galaxy string lay 10,800 million light-years away. Light travels almost 9.5 trillion kilometres in one light-year, so our observation of the string is as it appeared 10.8 billion years ago. The universe was formed during the Big Bang approximately 13.7 billion years ago.

"We have detected 37 galaxies and one quasar in the string, but it probably contains many thousands of galaxies," Dr Francis said.

"The really exciting aspect of this finding is that it sheds new light on the formation of the universe. We are looking back four-fifths of the way to the beginning of the Universe and the existence of this galaxy string will send astrophysicists around the world back to the drawing board, to re-examine theories of the formation of the Universe."

The string was discovered by Dr Francis, Dr Povilas Palunas of the University of Texas, Dr Harry Teplitz of the California Institute of Technology, Dr Gerard Williger of Johns Hopkins University and Dr Bruce E. Woodgate of NASA Goddard Space Flight Center, using telescopes in Chile and at Siding Spring Observatory in New South Wales.

The team were refused time on a US telescope because many American astronomers believed the observations were technically impossible. The findings have been presented at the American Astronomical Society meeting in Atlanta.

The team compared their observations to supercomputer simulations of the early Universe, which could not reproduce strings this large. "The simulations tell us that you cannot take the matter in the early Universe and line it up in strings this large," Dr Francis said.

"There simply hasn’t been enough time since the Big Bang for it to form structures this colossal.

"All we are seeing is the brightest few galaxies. That’s probably far less than 1 per cent of what’s really out there, most of which is mysterious invisible dark matter. It could be that the dark matter is not arranged in the same way as the galaxies we are seeing."

Recently, evidence has accumulated for the presence of dark matter in the Universe, an invisible form of matter only detectable by the gravitational pull it exerts on ordinary matter (and light). There are many possibilities for what dark matter might be, but its true nature is currently unknown.

In recent years, it had been found that in the local Universe, dark matter is distributed on large scales in very much the same way as galaxies are, rather than being more clumpy, or less. But go back 10 billion years and it could be a very different story. Galaxies probably form in the centre of dark matter clouds. But in the early Universe, most galaxies had not yet formed, and most dark matter clouds will not yet contain a galaxy.

"To explain our results the dark matter clouds that lie in strings must have formed galaxies, while the dark matter clouds elsewhere have not done so. We’ve no idea why this happened – it’s not what the models predict," Dr Francis said.

The astronomers say the next step is to map an area of sky ten times larger, to get a better idea of the large-scale structure. Several such surveys are currently under way. The research was funded by NASA and the Australian National University.