Buried in the faint microwave afterglow of the Big Bang, a nearly uniform sea of radiation covering the entire sky, astronomers found one patch roughly 10 degrees across that's colder than it has any statistical business being. For nearly two decades, the leading explanation seemed close to settled when researchers confirmed a genuinely enormous empty region of space sitting in exactly that direction. Then, in 2025, more careful analysis found that void can only account for a small fraction of the missing heat, leaving one of cosmology's most persistent anomalies still very much open.
Background: A Cold Patch in the Afterglow of Creation
The cosmic microwave background, or CMB, is relic radiation left over from roughly 380,000 years after the Big Bang, when the universe had cooled enough for light to travel freely for the first time. It fills the entire sky at an almost perfectly uniform temperature of 2.725 Kelvin, with tiny fluctuations, differences of only about one part in 100,000, mapping the primordial density variations that eventually seeded every galaxy and cluster in the universe. In 2004, using a specialized wavelet filtering technique applied to data from NASA's WMAP satellite, astronomers identified something unusual: a patch of sky in the direction of the constellation Eridanus, roughly 10 degrees in diameter, running about 70 millionths of a degree colder than the CMB's already frigid average temperature. The finding was later confirmed independently by the European Space Agency's Planck satellite, ruling out both measurement error and ordinary contamination from our own galaxy's foreground dust as explanations.
The Void Explanation
The leading theoretical mechanism for such a cold patch is called the Integrated Sachs-Wolfe effect: as CMB photons travel across billions of light-years toward Earth, passing through an unusually large, empty region of space, or supervoid, the accelerating expansion of the universe driven by dark energy can cause those photons to lose a small amount of energy, making that patch of sky appear very slightly colder than its surroundings. In 2015, a team led by astronomer Istvan Szapudi at the University of Hawaii, using the Pan-STARRS1 telescope and NASA's WISE infrared survey, identified a candidate supervoid roughly 1.8 billion light-years across, located about 3 billion light-years from Earth, directly in the Cold Spot's direction. In December 2021, the Dark Energy Survey went further, confirming the existence of a roughly 1-billion-light-year-wide void in that same region using detailed galaxy survey data, a structure now known as the Eridanus supervoid.
The 2025 Complication
For a while, the confirmed Eridanus supervoid looked like the answer. Then, in 2025, a more rigorous analysis led by astrophysicist András Kovacs, drawing on detailed Dark Energy Survey modeling, found that the confirmed void could only account for roughly 10 millionths of a degree of the Cold Spot's cooling, a fraction of the full 70-millionths-of-a-degree signal that needs explaining. As Kovacs put it, having both a genuinely rare cold spot and a genuinely rare supervoid overlapping in the same direction isn't, on its own, enough to prove one caused the other by the usual scientific standard. If the standard cosmological model is correct, the Cold Spot may simply be an extreme statistical outlier that happens to coincide with an unrelated void. If the standard model is instead missing something, the mismatch could point toward a genuine gap in current understanding of how dark energy interacts with light traveling through the largest structures in the universe.
Cosmic Anomalies at a Glance
| Anomaly | What's Observed | Leading Explanation | Status |
|---|---|---|---|
| Fast Radio Bursts | Millisecond-long, extremely energetic radio pulses from deep space | Magnetar activity, confirmed for at least one source | Active research; likely multiple distinct causes |
| Planet Nine | Unusual clustering in the orbits of distant icy solar system objects | An undiscovered large planet in the outer solar system | Unconfirmed; new survey telescope now searching |
| The Great Attractor | Thousands of galaxies, including our own, drawn toward one region | A massive concentration of galaxy clusters, hidden behind the Milky Way | Partially mapped; part of an even larger structure |
| CMB Cold Spot | An unusually cold, statistically unlikely region in the cosmic microwave background | A confirmed supervoid, though it's too small to fully explain the effect | Debated; the leading explanation is only partial |
| Axis of Evil (CMB Anomaly) | Unexpected large-scale alignment in cosmic background radiation patterns | Possible instrument or foreground contamination | Unresolved; challenges assumptions about cosmic randomness |
Theories and Explanations
The Integrated Sachs-Wolfe effect, partially confirmed. The Eridanus supervoid is real, confirmed by independent survey data, and sits in precisely the right location to contribute to the Cold Spot through the ISW effect. The problem isn't whether the mechanism is real; it's that the confirmed void appears too small to produce the full magnitude of cooling actually observed.
A statistical fluke under the standard model. If the standard Lambda-CDM cosmological model is entirely correct, an anomaly this unlikely could still occasionally occur somewhere in the sky purely by chance, a rare but technically permitted outlier rather than evidence of anything genuinely wrong with the model.
A gap in dark energy physics. If the confirmed void truly can't account for the full signal, some researchers see that mismatch as a potential hint that current models of how dark energy affects light traveling through cosmic voids may be incomplete in ways not yet well understood.
Contamination from nearby galaxies. A separate line of research published in 2023 and 2024 identified an unexplained, systematic dip in apparent CMB temperature specifically around certain large, nearby spiral galaxies, an effect whose underlying physical cause remains unclear but which some researchers are investigating as a possible contributing factor across several CMB anomalies, the Cold Spot potentially included.
The Curious Connection
The CMB Cold Spot follows almost exactly the same arc as this series' previous entry on the Great Attractor: a leading explanation that appeared to close the case, followed by more careful analysis revealing the resolution was only partial, uncovering a deeper layer of the mystery underneath. Finding the Norma Cluster didn't fully explain the Great Attractor; finding Laniakea revealed it was itself being pulled toward something larger. Confirming the Eridanus supervoid didn't fully explain the Cold Spot either; more rigorous modeling revealed it accounts for only a fraction of what needs explaining.
That's now two consecutive entries in this series where the story isn't really "unsolved" so much as "solved incompletely, in a way that keeps generating new questions." This series' final entry turns to a related anomaly drawn from the very same WMAP and Planck datasets that first revealed the Cold Spot, one that raises the more unsettling possibility that several of these separate cosmic anomalies might not need five different explanations at all, but could instead share one common underlying cause no one has confirmed yet.
Frequently Asked Questions
What is the CMB Cold Spot?
The CMB Cold Spot is an unusually large and statistically unlikely cold region, roughly 10 degrees in diameter, found in the cosmic microwave background, the relic radiation left over from the early universe, located in the direction of the constellation Eridanus.
What causes the CMB Cold Spot?
The leading theory involves the Integrated Sachs-Wolfe effect, in which CMB light loses energy passing through a large, unusually empty region of space called a supervoid. A confirmed supervoid exists in the right location, but 2025 research found it's too small to fully explain the observed cooling.
What is the Eridanus supervoid?
The Eridanus supervoid is a confirmed region of space roughly 1 billion light-years across containing far fewer galaxies than average, identified by the Dark Energy Survey in 2021 in the same direction as the CMB Cold Spot.
Is the CMB Cold Spot fully explained now?
No. While the Eridanus supervoid is confirmed to exist, 2025 analysis found it can only account for roughly one-seventh of the Cold Spot's total temperature deficit, leaving the anomaly only partially explained.
Could the CMB Cold Spot just be a random statistical fluctuation?
It's possible. If the standard cosmological model is correct, an anomaly this size could occur by chance somewhere in the sky, though its coincidental overlap with a confirmed supervoid has kept researchers actively investigating alternative explanations.