More than ten years ago, while taking the temperature of the universe, astronomers found something odd. They discovered that a patch of sky, spanning the width of 20 moons, was unusually cold.

The astronomers were measuring the microwave radiation that bathes the entire universe, a glowing relic of the big bang. To gaze at this cosmic microwave background, or CMB, is to glimpse the primordial universe, a time when it was less than 400,000 years old.

What's now emerging as the top hypothesis is a cosmic supervoid

The CMB blankets the sky, and looks pretty much the same everywhere, smoldering at a feebly cold temperature of 2.725 kelvins - just a couple degrees warmer than absolute zero. But armed with the newly launched WMAP satellite, the astronomers had set out to probe temperature variations as tiny as one part in 100,000. Born from the quantum froth that was the universe a half-moment after the big bang, those random fluctuations help scientists understand what the cosmos is made of and how it all came to be.

And standing out amidst those fluctuations was a cold spot. Over the years, astronomers have come up with all sorts of ideas to explain it, ranging from instrumental error to parallel universes. But now, they're homing in on a prime suspect: an enormous cavern of emptiness called a cosmic supervoid, so big that it might be the largest structure in the universe.

According to theory, such a vast void, in which nary a star or galaxy exists, can leave a frigid imprint on the CMB. The answer to the mystery, then, might simply be a whole lot of nothing. Yet puzzles remain, and the case is far from closed.

How to make a cold spot

The cold spot isn't the only weird thing in the CMB. Scientists have found several other such anomalies - for example, the signals from half the sky appear slightly stronger than the other half. The standard theory of cosmology, which has otherwise been prophetic in predicting the CMB's details, can't fully explain these oddities, of which the cold spot is one of the most prominent.

A really big one could act as a kind of distorting lens

The simplest explanation for the anomalies is that they're flukes, artifacts of chance among the random temperature fluctuations of the CMB. When you flip a coin a hundred times, there's always a chance you get 20, 30, or even 50 heads in a row. The challenge for scientists is to figure out whether those anomalies are due to luck or a weighted coin. As for the cold spot, the data shows that the likelihood it's a fluke is one in 200. Not impossible, but not likely, either.

Some scientists had suggested the cold spot was due to instrumental error or in the way the data was analysed. But in 2013, new observations from the Planck satellite confirmed earlier detections of the cold spot. And it demanded an explanation.

What's now emerging as the top hypothesis is a cosmic supervoid. All the stuff in the cosmos - galaxies and invisible dark matter - stretches across space in a vast web of sheets, tendrils, and filaments. In between are pockets of emptiness called voids, which come in many shapes and sizes. A really big one could act as a kind of distorting lens, making the CMB appear cooler than it really is.

While a photon chugs along inside a void, the universe continues to expand faster and faster

The reason is this: When light travels through a void, it loses energy and its frequency decreases, shifting towards the lower frequency, redder end of the spectrum. Like most things, light is susceptible to the influence of gravity, which can act on photons along their journey. Inside a void, however, the dearth of matter means there's hardly any gravity to influence the light. For a photon, flying through a void is like climbing over a hill. And climbing requires energy.

But the photon can get that energy back. Once it exits the void, it finds itself surrounded with matter again, and the gravitational influence is enough to pull on it, injecting it with the energy it had lost.

For a photon to lose energy, you need the accelerated expansion of the universe. While a photon chugs along inside a void, the universe continues to expand faster and faster. By the time the photon leaves the void, it finds that - thanks to this cosmic stretching - all the matter has spread out. Because the stuff is now more widely distributed, its gravitational effect isn't as strong. It can't pull on the photon with the same strength as it did before, and the photon can't recover the energy it once had.

There might be a void less than 3 billion light years away

Physicists worked out this phenomenon back in the late 1960s, but no one had actually observed it. But after the cold spot was discovered, astronomers such as Istvan Szapudi of the University of Hawaii started searching for evidence of this behaviour, called the integrated Sachs-Wolfe, or ISW, effect. In 2008, he found it.

The amazing supervoid

Szapudi couldn't identify individual voids leaving behind imprints on the CMB - he didn't have the data to do that. Instead, he and his team searched for an overall ISW effect in a statistical analysis of 100 voids and galaxy clusters, whose gravitational heft creates a warming effect and leaves hot spots in the CMB. The researchers found a real ISW effect, changing the temperature of the CMB by an average of about 10 millionths of a kelvin, or 10 microkelvin.

Compared to the cold spot, which is about 70 microkelvin cooler than the CMB's average, the effect is small. But the point was to show that voids could create cold spots. If a void were big enough, it could conceivably create the cold spot. "If this cold spot is the biggest anomaly in the CMB, it could very well be a sign of a huge void - a very rare void in the universe," Szapudi says. "So I thought we should now look for it."

The void is huge. It's 220 megaparsecs in radius

His first attempt, in 2010, turned up empty. But the data was limited, covering only a few points within the spot. Intriguingly, the results also showed that there might be a void less than 3 billion light years away.

Last year, he and his team tried again, this time with loads more data, covering over 200 times more sky and encompassing the entire cold spot. With so much more coverage - consisting of thousands of galaxies - those earlier hints coalesced into a bona fide void. The data was unequivocal. "We're absolutely sure there is a void," Szapudi says. "I would bet my house on it."

And the void is huge. It's 220 megaparsecs in radius, more than 700 million light years, which makes it one of the biggest - if not the biggest - physical structures in the universe.

Such a large void is uncommon, with maybe only a handful in existence, Szapudi says. That such a rare void overlaps the cold spot - itself another rarity - seems too unlikely to be mere coincidence. What's more likely, he says, is that the void is causing the cold spot. In fact, he calculated that scenario to be 20,000 times more probable than if the two objects had just happened to align.

Others aren't yet sure. For astronomers such as Patricio Vielva of the University of Cantabria in Spain, who led the discovery of the cold spot in 2004, the rarity of the void is still in question. If it turns out that such voids are more widespread, then this alignment wouldn't be so remarkable. Maybe it is just a coincidence. Which is why researchers need more data to gauge how rare these supervoids are. "Right now, I think this is one of the most important things to establish," Vielva says.

Not cold enough

But there's a bigger problem.

The supervoid can't get the CMB cold enough. A supervoid of this size can only cool the CMB by 20 microkelvin. The cold spot, however, is on average colder by 70 microkelvin. At some points, the temperature drop is 140 microkelvin.

One possible reason behind the discrepancy is that the void is actually larger than measured. If so, its ISW effect would be stronger. Given the uncertainties of Szapudi's measurements, the void's radius could stretch as far as 270 megaparsecs. Still, Vielva says, even that's not big enough to account for the cold spot.

In fact, according to current theories of cosmology, the universe may not even be able to form a void that is big enough. "The problem is that the kind of void you need for this effect is nonexistent," Vielva says.

More observations will allow astronomers to get more accurate measurements of the supervoid's size and properties

But if not a void, then what? Perhaps, Vielva says, the cold spot is due to a cosmological texture, a defect in the universe analogous to the cracks or spots found in ice. As the early universe evolved, it underwent a phase transition similar to what happens when water freezes, turning from liquid to solid. In ice, you get defects when the water molecules don't line up. In the universe, you might get textures. In 2007, Vielva helped show that if a texture exists, it could create the cold spot via the ISW effect.

Textures, though, are speculative, and no one has seen any evidence that they exist. "Textures are a nice idea, but we have no clue as to whether these things are realistic or not," says Rien van de Weijgaert, an astronomer at the University of Groningen in the Netherlands.

For most astronomers, van de Weijgaert says, a supervoid still seems the best explanation. "By now, it's considered to be one of the most believable options," he says. "It's the magnitude of the effect that you could have some questions about, but it's not unbelievable."

To be sure, the void hypothesis is certainly intriguing, Vielva says. But the temperature discrepancy must first be resolved.

We just don't know the end of the story. I don't think anybody knows

More data would help. For instance, more observations will allow astronomers to get more accurate measurements of the supervoid's size and properties. They might also reveal whether there's a smaller void in the foreground, which could help cool the CMB. Perhaps, the cold spot is so frigid because the supervoid also happens to be in front of a region of the CMB that's already a bit colder than normal.

Even though the numbers don't add up now, it's no reason to fret. "At this point, because the uncertainties are so large, one should not lose much sleep over this," says Carlos Frenk, an astrophysicist at the University of Durham in the UK. His hunch is that with more data and analysis, the supervoid will emerge as the correct answer. "It could very well be that it all falls into place quite neatly," he says.

If so, the cold spot will represent the first measurement of an object - a supervoid - leaving an imprint on the CMB via the ISW effect. That's significant partly because the supervoid is simply so huge. The supervoid could be important in another way: "We have one more way to study dark energy, which is the weirdest thing in the universe," Szapudi says.

The ISW effect only works because the universe expands faster and faster, and the mysterious force pushing the cosmos apart is dark energy. By measuring the ISW effect from the supervoid, researchers can probe dark energy's influence - and better understand how it behaves and what it is.

But for now, the mystery of the cold spot continues. "We just don't know the end of the story," Frenk says. "I don't think anybody knows."