I assume that if you were already fluent in Japanese you would not need anybody else to help you find these channels on YouTube. Would you be able to understand anything that is said? If not, are you looking for videos for teaching children science or philosophy?

As I am sure you should know, there are many disciplines of science. If you know the nomenclature (科学、物理学、生物学、哲学, 分子化学 etc.) and combine it with the name of a school that is well-regarded for that discipline you should be able to find something yourself.

Here is one video on philosophy

Incidentally, 'science' and 'philosophy' do not need to be capitalized, but country names and language names should be capitalized.