system
Given the following post, respond as if you're commenting on Reddit:
human
What is the strongest evidence for the existence of the Oort Cloud?

I have been looking into this and keep hearing scientist saying they believe that the Oort cloud does exist.  That word keeps standing out to me and can't see its place in the situation.  I am interested in the evidence for the cloud.  Not an explanation of what such a cloud would be or someones personal feelings about it.
gpt
Evidence in solar system dynamics is usually given of the form "if this has happened, then this must have happened"

If we see a huge crater with an ejecta blanket around it, we must be looking at the site of an ancient impact, for example. This isn't an explanation of what an impactor would be or anyone's personal feelings.

What we see in evidence for the Oort cloud is from several avenues of investigation. We know short period comets, those which have a period of less than 200 years, tend to be aligned with the plane of the solar system (their inclination is low). They did not come from a cloud, but instead from a belt - the Kuiper belt.

Long period comets, those we see in thousand-year orbits, come from random parts of the sky, their inclination is random. Knowing a longer orbital period is a much more distant aphelion, and the comets move more slowly at aphelion, so long period comets will dwell at aphelion, so forming the appearance of a cloud of them. This is fairly basic orbital mechanics, and very well understood.

The next bit of evidence comes from Late Heavy Bombardment. This was discovered during the Apollo missions, although the Soviets learned about it from their robotic sample returns too. The age of the oldest lunar material was significantly younger than anyone could account for. It was 3.8 billion years old, and nobody could tell us why.

This was the case until the Nice models, which showed that, in the early solar system, Uranus and Neptune swapped places during a period of instability caused by Jupiter and Saturn in a 2:1 resonance. Neptune crashed through the early (and very dense) Kuiper belt, scattering everything around, losing its own moons, capturing Triton, capturing Pluto and the plutinos, forming the scattered disc... Yet interactions from Neptune would indeed scatter objects around, and even to very high aphelia, but would only rarely give them enough energy to escape, so these highly scattered objects remained gravitationally bound. 

Couple these scattered objects with the ones scattered during plantetary formation, then add in the effects of galactic tides, and their mutual gravity, and we get a slow, but steady, decline in aphelia and rise in perihelia: Their orbits tend to drift toward being circular. This, then, is the Oort cloud.

You won't see any physical evidence, there are no stone tablets or walking trees saying "I am Oort". The formation happened billions of years ago and we can't time travel.

What we can do, however, is predict its existence and then go to find it. Sometimes this happens quickly: Le Verrier predicted Neptune, and it was rapidly found. Sometimes slowly: Gerard Kuiper had an inkling that there may be a belt just outside Neptune (he didn't do any dynamics work on it, though), from early work done by Leonard (1930), Edgeworth (1943) and Cameron (1962). Nobody actually found one until 1992, when 1992 QB1 was discovered and, six months later, 1993 FW.

Oort cloud objects will be significantly more difficult to observe than the Kuiper belt is.