When multiple voids identified in galaxy surveys are stacked together to create a composite void profile, the resulting temperature structure shows a systematic cold bias—the voids appear colder (lower temperature in the Cosmic Microwave Background) when stacked than would be predicted by Lambda-CDM simulations based on the matter underdensity alone. This cold bias suggests either that voids have intrinsic properties not captured by simple matter-underdensity models, or that the relationship between matter distribution and CMB temperature is more complex than standard models predict. The stacking analysis finds that voids are statistically colder than expected, indicating either that void formation processes differ from Lambda-CDM predictions or that some additional physical mechanism (beyond gravitational evolution) contributes to the temperature structure (Granett et al. 2010; Nadathur and Hotchkiss 2015).