The last 18 months have been tense for the Nasa astronomer Dr Sasha Kashlinsky and his colleagues. They have been scouring the heavens in search of more evidence for their astonishing claim, made in 2008, that they have found signs of a universe beyond our own. As discoveries go, this would rank alongside proof that the Earth goes round the Sun and that the universe began with a big bang. It would change our beliefs in the nature of reality. So perhaps not surprisingly, there has been considerable scepticism from the rest of the scientific community. Some insist the team has been fooled by some subtle trick of nature, or has simply made a mistake.
But now we may just have to get used to the idea that there really is more to the universe than, well, the universe. Dr Kashlinsky and his colleagues have unveiled the extra evidence they were seeking, and it continues to point to the existence of something beyond our universe. The evidence takes the form of what the team calls the Dark Flow, which they discovered during what should have been a routine bit of research. According to current theories of the universe, there should be a certain amount of "lumpiness" in the distribution of matter throughout space, left over from the Big Bang around 14 billion years ago.
The gravitational fields of these vast lumps of primordial matter should reveal themselves in random motions of clusters of galaxies, which would otherwise simply recede from each other as the universe expands. It was these random motions that Dr Kashlinsky and his colleagues were scrutinising when they stumbled across something far more dramatic. Analysis of the motion of more than 700 clusters of galaxies revealed that the motion of many of them is anything but random.
A colossal region of the cosmos spanning a billion light years of distance seemed to be filled with some invisible current, carrying the galaxies away at more than 3 million kilometres per hour. They all appeared to be heading in the direction of the constellations Centaurus and Hydra, but exactly what was coaxing them out of our universe remained invisible. Because proving this discovery depended on teasing out very subtle effects from a morass of data, many astronomers immediately suspected some kind of statistical fluke. That is now looking increasingly unlikely. Results published last month by Dr Kashlinsky and his team in the prestigious Astrophysical Journal Letters and based on twice as many galaxy clusters still show the Dark Flow effect - which now extends out to 2.5 billion light years.
So what is causing this cosmic current? One possibility put forward is that there is nothing mysterious pulling the galaxy clusters at all, but rather a lack of matter in our part of the universe - and thus inadequate gravitational force to restrain them as they are attracted towards matter in the opposite direction. Yet while that would create a kind of Dark Flow effect, the researchers insist it can't explain the fine detail of their observations. They still maintain that the best explanation is that the clusters of galaxies really are being affected by something lying outside our universe.
At first sight, this is preposterous: if the universe really is everything, how can there be anything "beyond" it? In part, the answer lies in knowing exactly what is meant by the term "universe". While it may indeed encompass literally everything, astronomers have long known that we will only ever see a tiny part of it - because of the way the universe expands. Viewed from any point within the universe, the most distant galaxies appear to be racing away from each other, their speed of recession increasing with their distance.
Predicted in the 1920s and discovered shortly afterwards, this is the so-called Hubble Law of cosmic expansion, and it implies the existence of a distance at which galaxies are receding so fast their light can never reach us. They may still be in our universe, but they are no longer in the visible universe - which current observations suggest is more than 80 billion light years across. And this is where Dr Kashlinsky and his colleagues think the cause of the Dark Flow is lurking. Somewhere out there, beyond our cosmic horizon tens of billions of light years away, lies more matter - the gravity of which is dragging whole clusters of galaxies from our part of the cosmos.
Despite the new findings, many astronomers remain sceptical. They point out that even if the Dark Flow has been detected out to around 2.5 billion light years, this is still barely five per cent of the distance to the cosmic horizon. They want to see the Dark Flow continuing far deeper into space before countenancing the idea of effects from beyond our universe. The evidence they demand may soon be at hand, thanks to an orbiting observatory launched last year by the European Space Agency. Known as Planck, it is about to complete the most thorough survey of the heat left over from the Big Bang. By mapping out the spread of that heat - and thus matter - across the universe in unprecedented detail, Planck will reveal more details about the Dark Flow - if it exists.
Around 2,000 years ago, the Roman philosopher Lucretius imagined standing at the edge of the universe and throwing a spear into the void beyond. Either the spear would travel on, or it would hit a barrier at the edge and bounce back. Either way, argued Lucretius, it would imply there was something beyond the supposed edge - thus proving the universe must be infinite. Lucretius never reckoned on the possibility of something reaching out from beyond the other side and snatching the spear from his hand.
Robert Matthews is Visiting Reader in Science at Aston University, Birmingham, England