An image of the distribution of GRBs on the sky at a distance of seven billion light years, centred on the newly discovered ring. The positions of the GRBs are marked by blue dots and the Milky Way is indicated for reference, running from left to right across the image. L. Balazs

Astronomers are puzzled over what is creating the largest feature in the observable universe -- a massive sphere of nine galaxies across five billion light-years.

The galactic ring revealed by nine gamma-ray bursts (GRBs), is located seven billion light-years away and in a circle more than 70 times the diameter of a full moon.

It contradicts current models of the universe.

All nine are at similar distances from Earth and are clearly associated with the same structure, says Lajos Balazs of Konkoly Observatory in Budapest who led the study.

However, this structure is at variance with the Cosmological Principle that says the universe has a uniform distribution of matter over the larger scale and sets a limit of 1.2 billion light-years.

The present structure is five times larger than that limit.

Observations of the early universe and the background cosmic microwave radiation by Nasa's Wilkinson Microwave Anisotropy Probe (WMAP) and Europe's Planck space telescope have confirmed the principle.

Instead of being a real spatial structure, the ring could be a projection of a sphere, where the GRBs all occurred within a brief 250 million year period, said Balzacs.

Alternate models of the universe have suggested clusters of galaxies coming together in a web-like structure around concentrations of dark matter. Such a sphere would then mirror strings of galaxy clusters around the voids or empty space between filaments.

The present ring of galaxies is at least ten times larger than known voids.

Astronomers will now study possible ways in which the present ring could have formed, failing which cosmological models will need to be replaced.

GRBs are the explosions accompanying the death of a star.

The work is published in a paper in Monthly Notices of the Royal Astronomical Society.