A frequent travel spot for Leonardo, Monte Rosa is a large mountain in the Swiss Alps, very close to the Swiss-Italian Border.  At the peak, da Vinci noticed the extreme blueness of the sky,  commenting that "If you go to the top of a high mountain the sky will look proportionally darker above you as the atmosphere becomes rarer between you and the outer darkness; and this will be more visible at each degree of increasing height till at last we should find darkness." He speculated that the blue effects was due to the humidity in the air catching the rays of the sun (i.e. the same process which gives water its blue tint). While not completely correct, da Vinci shows a scientific mind well ahead of his time, as it took until 1871, when Lord Rayleigh published a paper depicting this phenomenon as the quantum interaction between particles and radiation. In this way, da Vinci unknowingly also explains the reason outer space is dark, despite stars shining light throughout it: there needs to exist matter within the gaps to reflect it or the main regions will appear dark. 

Sources:

Isaacson, W. (2018). Leonardo da Vinci: the biography. London: Simon & Schuster.

The Editors of Encyclopaedia Britannica. (2018, February 12). Rayleigh scattering. Retrieved February 4, 2020, from https://www.britannica.com/science/Rayleigh-scattering