The Eridanus supervoid is an enormous underdensity in the galaxy distribution spanning roughly 300 Mpc in radius, about two billion light-years across, toward the constellation Eridanus, identified through galaxy counts and later through dedicated tracer surveys (Granett et al. 2010; Szapudi et al. 2015). Underdensities of this scale and depth sit far out on the statistical tail of what Gaussian initial conditions grown by standard gravity produce; finding one would be notable, and the local universe holds several.
The void carries a second, sharper problem: it sits in the direction of the CMB Cold Spot. The natural reading, that the supervoid imprints the Cold Spot through the integrated Sachs-Wolfe effect, fails quantitatively within ΛCDM, because the ISW imprint of even this void at standard potential-decay rates is several times too shallow to produce the observed temperature deficit (Szapudi et al. 2015; Mackenzie et al. 2017). The model is left holding two improbable objects, a statistically rare supervoid and an unexplained Cold Spot, aligned on the sky by what must then be coincidence, with the mechanism that should connect them officially too weak to do so.
The standing is doubly uncomfortable and actively studied. Deeper tracer surveys keep confirming the void's reality while refining its profile, and the alignment question, fluke or physics, will be settled by the void censuses and ISW environment-splits that DESI and Euclid are now assembling.