For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some ‘organic element’ since I couldn’t accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
Not too sure where you got that number from. From what I can find, the radius of the observable universe is estimated to be about 46.5 billion light-years.
Edit: I see now that you are talking Galaxy. That’s different.
The original comment was about stars we can see in the sky, so I was assuming naked eye