Re: the still undiscovered Oort Cloud?
"Does he mean we never observed an object in the Oort Cloud, yet we know it exists?"
Pretty much. We know that starbirth is likely to produce a loose 'shell' of gravitational objects within a region beyond the planetary system, and that there is evidence of orbital perturbation on comets that enter the inner system.
It's rather like the early part of the 20th Century, when astronomers were looking for a 9th planet, which had never been spotted but was obviously there based on its gravitational effects.
Biting the hand that feeds IT
