The cosmic microwave background (CMB), the light echo from the Big Bang, has many puzzling features but none as mysterious as the "Cold Spot", a feature that appears much colder than the rest of the CMB in the sky.

It has been traditionally explained as a supervoid, a region billions of light-years across, with only a relatively small number of galaxies inside. But new research, published in the Monthly Notices of the Royal Astronomical Society, suggests that this explanation is incorrect.

Researchers, led by a team from Durham University, measured the precise distance of 7,000 galaxies in the direction of the Cold Spot and in other regions of the sky. Their observations allowed them to construct 3D slices of the universe, which showed that there is nothing abnormal in the number of galaxies towards the Cold Spot. 

The CMB is made up of the original photons from the Big Bang. As the universe expands, it causes the light from distant galaxies to redshift, meaning the photons are moved toward redder (longer) wavelengths, in this case from visible light to infrared and now microwaves. While this is the strongest effect we see on the CMB there’s also a more subtle one called the Sachs-Wolfe effect, a gravitational redshift due to matter distribution being uneven. 

By measuring galaxies' redshift we can measure distances. What the researchers found was that galaxies towards the Cold Spot were distributed in the expected soap bubble arrangement known as the cosmic web, with galaxy clusters alternating with small voids, just like anywhere else in the sky.

"The voids we have detected cannot explain the Cold Spot under standard cosmology," lead author Ruari Mackenzie said in a statement. "There is the possibility that some non-standard model could be proposed to link the two in the future but our data place powerful constraints on any attempt to do that."

The theory of the Cold Spot as a supervoid was already pretty unusual and pushing the limit of the standard model, which says the universe is more or less the same in every direction. With this new finding, it might mean there’s a different explanation out there. For example, simulations give a 2 percent chance of the Cold Spot having actually formed by chance.

"This means we can't entirely rule out that the Spot is caused by an unlikely fluctuation explained by the standard model. But if that isn't the answer, then there are more exotic explanations,” added co-author Professor Tom Shanks.

One of these possible explanations has the Cold Spot as the scar of a cosmic collision between our universe and another. So far, we are only getting a picture of what the Cold Spot is not, better data might one day tell us what it is.