Galaxies are not distributed evenly throughout the universe. Like the Local Group, they tend to be gathered in clusters, which are separated from each other by voids. The bigger the void, the rarer it should be. The problem with the great void is that it is so big it shouldn't exist. Its size challenges the standard inflation model of how the universe developed after the big bang. Laura Mersini-Houghton, of the University of North Carolina, caught the attention of the world's cosmologists with a radical theory — radical to the layman perhaps — but in the bizarre world of quantum mechanics and big bang inflation theory, actually quite sensible.

"It is the unmistakable imprint of another universe beyond the edge of our own," she told New Scientist. "Any fluctuation leading to a void as big as (this) is exceedingly unlikely, according to standard cosmology." Dr Mersini-Houghton's theory is based on the "multiverse" concept, the increasingly accepted idea that our universe is just one of many. In the first split-second of its existence after the big bang, our universe and its neighbours were each the size of a sub-atomic particle. Objects of that size are subject to quantum entanglement — they can affect each other's properties even if they are physically separated. Our tiny universe entangled itself with a neighbouring universe, the result of which is the great void. "Such an entangled state remains for all time," Dr Mersini-Houghton told New Scientist. "So although inflation quickly pushed our region beyond the reach of neighbouring regions, it should still retain the imprint of its quantum entanglement with its neighbours … we are amazed that the void is there just as we predicted."

The void is about 6-10 billion light years away in the constellation of Eridanus, which is visible from Melbourne. Cosmologists believe that the tiny imperfections in the microscopic universe are directly reflected in the large-scale structures of galactic clusters and voids that we see today, so the idea of something sub-atomic causing a billion light-year void is actually quite reasonable. But no models of the universe's expansion predict a void as big as this.

"What we've found is not normal, based on either observational studies or on computer simulations of the large-scale evolution of the universe," said co-discoverer Liliya Williams, from the University of Minnesota. Dr Mersini-Houghton's entanglement theory offers a a way around this. Nevertheless, she's had a mixed reception among cosmologists. "If it (the void) is real, then it could be the sign of something like this (entanglement) happening during the birth of the universe," said astronomer Brian Schmidt, from the Australian National University. "But it could have a multitude of other explanations … because we only get one look at the universe, so it is really hard to say if what we are seeing is really crazy, or just a statistical fluctuation."

Karl Glazebrook, from the Centre for Astrophysics at Swinburne University, said the statistical evidence for the void was still in question. "(The discoverers) have only looked at radio galaxies. There could be problems with having enough data, there could be problems with the statistics." Professor Glazebrook said he did not accept the explanation that the void was evidence of another universe.