The CMB Cold Spot (CMB stands for Cosmic Microwave Background) is a region of space in the direction of the Eridanus constellation that was first discovered in 2004 when scientists began measuring differences in the temperature of our universe's cosmic microwave background — the thermal radiation leftover from the Big Bang.

The region, which might best be described as a bubble of cold space bobbing in the cosmos, is massive. It represents one of the largest structures in the observable universe. And until recently, scientists had no explanation for it.

Of course, there have been no shortage of theories, and as with every great cosmic mystery, many of those theories have been wildly speculative. For instance, some have suggested the region might represent an imprint from a parallel universe, caused by quantum entanglement between the two universes before they were separated by cosmic inflation.

But now an international team of astronomers led by Istvan Szapudi of the Institute for Astronomy at The University of Hawaii at Manoa thinks they may have finally zeroed in on a more sensible answer. They have found evidence for what is known as a "supervoid," an area where the density of galaxies is much lower than usual, that exists between us and the primordial cold spot, reports Discover.

To understand how a supervoid might create the cold spot, imagine yourself as a photon from the CMB passing through such a void. Because the universe is constantly expanding, when you exit the void you will have entered into a medium that is less dense than the medium from before the void. Lower density means weaker gravitational pull, which means you won't be able to make up all the energy you lost. Less energy translates to a lower temperature, thus the cold spot.

To confirm the existence of such a supervoid, Szapudi and his team used NASA’s Wide Field Survey Explorer (WISE) and the robotic telescope Pan-STARRS1 to generate a tomographic map of the distribution of galaxies in the cold spot. Sure enough, they found a dip in the numbers, consistent with the presence of a supervoid.

But calling this void "super" might actually be an understatement. To create such a large cold spot, the supervoid has to be even more mammoth. Scientists estimate that it must have stretched 1.8 billion light years across the sky when the universe was 11.1 billion years old, making it the single largest known structure in the universe.

The discovery is exciting, if not mind-bending, but there are still questions to be answered. Measuring things on such a large scale also means that errors are more likely. So more studies must be performed to be certain of the findings. Ultimately, though, the research promises to expand our understanding of voids, and in the process enhance our cosmological theories too.

It just goes to show that when it comes to space, even vast regions of emptiness can still be full of mystery and wonder.