The galaxy cluster 1E 0657-56, 3.8 billion light-years away, is one of hundreds that appear to be carried along by a mysterious cosmic flow (Image: NASA / STScI / Magellan / U.Arizona / D.Clowe et al)

WE CANNOT see what lies beyond the visible horizon of our universe, simply because light emitted beyond that horizon has not had time to reach us. Despite this out-of-sightness, we’ve always assumed that space is filled with the same stuff wherever you go in the universe.

So a recent finding by Sasha Kashlinsky at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, does not make sense. His team has found a group of galaxy clusters moving at an extraordinary speed towards a small patch of sky between the constellations of Centaurus and Vela. Kashlinsky calls it the “dark flow”, in tribute to those other cosmic mysteries dark matter and dark energy (New Scientist, 24 January, p 50).

There is no obvious reason why the clusters should be moving at such breakneck speeds, unless they are experiencing an unusually strong pull from something beyond the visible horizon. But what? The most obvious answer is that there is something big out there, far bigger than anything in our known universe. Such a behemoth would impose a kind of “tilt” on the universe, causing matter to move in one particular direction – as observations of the dark flow suggest.


Something outside the visible universe appears to be pulling on galaxy clusters. But what?

If such cosmic megastructures do exist, though, they merely replace one mystery with another. One of the foundation stones of cosmology is the Copernican principle, which says that there is nothing special about our region of the universe. So if there are megastructures beyond our horizon, there should be megastructures in our patch, too. We haven’t seen any.

There are also suggestions that the pull might be from another universe altogether. That would be good news for proponents of eternal inflation theory, which suggests that the universe should actually be composed of “mini-universes” that have bubbled off from one another.

Kashlinsky is preparing papers with further results. He says observations point the finger at megastructures beyond the horizon.

Read more: 13 more things that don’t make sense