The Cold Spot is a circular region roughly 5 degrees in radius in the southern sky where the cosmic microwave background runs about 70 microkelvin colder than the mean, surrounded by a hotter ring that makes the profile even less likely as a random fluctuation. First seen by WMAP and confirmed by Planck, it sits at roughly 3 sigma significance against Gaussian expectations: about a one-in-a-few-hundred accident for the best-fit ΛCDM sky. Of all the large-angle CMB anomalies it is the most visually striking and the most stubborn, having survived two decades of foreground, systematic, and look-elsewhere scrutiny.
The standard model's leading explanation has been measured and found wanting. The Dark Energy Survey confirmed the Eridanus supervoid in front of the Cold Spot: about 1.8 billion light-years across and 20 percent underdense, one of the largest structures known. A void imprints cold through the integrated Sachs-Wolfe effect, but in ΛCDM the measured size and depth deliver only about -10 to -20 microkelvin, 10 to 20 percent of the observed decrement. Closing the gap inside the model requires either modified gravity to amplify the ISW response, an exotic remnant such as a cosmic texture, or the assertion that spot and void coincide by chance, stacking one rare object on another. Each escape has been proposed; none has independent support.
The standing is therefore an open wound rather than a crisis: a 3 sigma feature does not refute a model by itself, but a persistent, structured, ringed cold feature aligned with the largest known supervoid, with the void supplying only a fifth of the signal, has no satisfying ΛCDM account. Polarization measurements over the region by future experiments (LiteBIRD, CMB-S4) can distinguish a primordial fluctuation, which carries correlated polarization, from a line-of-sight or formation-epoch cause, which does not.