Please don't use that quote. It's apocryphal; he almost certainly never said that.
It's not about how close we are to "knowing most things," but about how accessible new learning is. In a game of musical chairs, all the easy chairs are taken.
> The beauty and clearness of the dynamical theory, which asserts heat and light to be modes of motion, is at present obscured by two clouds. I. The first came into existence with the undulatory theory of light, and was dealt with by Fresnel and Dr. Thomas Young; it involved the question, how could the earth move through an elastic solid, such as essentially is the luminiferous ether? II. The second is the Maxwell–Boltzmann doctrine regarding the partition of energy.[0]
As I understand it, the two clouds were dealt with via special relativity and quantum mechanics.
[0] https://books.google.com/books?id=YvoAAAAAYAAJ&pg=PA363#v=on...