The Oort Cloud Hypothesis

The Oort Cloud is a theoretical, vast spherical shell of icy objects believed to surround our solar system far beyond Pluto. It is the hypothesized source of long-period comets that occasionally visit the inner solar system. While its existence is widely accepted, it remains unobserved directly due to its extreme distance and the faintness of its objects, making it one of the largest and least understood structures in our solar system.