The sky is everything, but not black. More like a grey.....ish kind of color. Actually your teacher was wrong, the sky is blue because it is absorbing other wavelengths more than it is absorbing the wavelength of the color blue. That's why it appears blue for us, because we see different wavelengths of light in different colors. But we just see a small spectrum of the wavelengths that exist, actually, there are infinite "sizes" of wavelengths that exist. If the wavelengths are shorter than the wavelengths of purple, we call it "Hyper-violet" and if the wavelengths are larger than the wavelengths of red, we call it "infra-red".

gg myself, I can't stop talking.