The main evidence for the existence of the Oort cloud s the trajectories of comets.
from wikipedia
In 1932, the
Estonian astronomer
Ernst Öpik postulated that long-period comets originated in an orbiting cloud at the outermost edge of the
Solar System.
[7] The idea was independently revived by
Dutch astronomer
Jan Oort as a means to resolve a paradox.
[8] Over the course of the Solar System's existence the orbits of comets are unstable and eventually
dynamics dictate that a comet must either collide with the Sun or a planet or else be ejected from the Solar System by planetary
perturbations. Moreover, their volatile composition means that as they repeatedly approach the Sun,
radiation gradually boils the volatiles off until the comet splits or develops an insulating crust that prevents further
outgassing. Thus, Oort reasoned, a comet could not have formed while in its current orbit and must have been held in an outer reservoir for almost all of its existence.
[8][9][10]
There are two main classes of comet, short-period comets (also called
ecliptic comets) and long-period comets (also called nearly
isotropic comets). Ecliptic comets have relatively small orbits, below 10 AU, and follow the
ecliptic plane, the same plane in which the planets lie. All long-period comets have very large orbits, on the order of thousands of AU, and appear from every direction in the sky.
[10] Oort noted that there was a peak in numbers of long-period comets with
aphelia (their farthest distance from the Sun) of roughly 20,000 AU, which suggested a reservoir at that distance with a spherical, isotropic distribution.
[10] Those relatively rare comets with orbits of about 10,000 AU have probably gone through one or more orbits through the Solar System and have had their orbits drawn inward by the
gravity of the planets.
[10]