Aplaying flexbox to the body element.

Hello everybody. I have a html document with https://vshare.onl/ nothing but a 200x200 px div. When I try to center it with flexbox it centers horizontally without a problem, but vertically it doesn't. Why does this happens? Shouldn't it center the box in the middle of https://discord.software/the screen? Or does the height of the body defaults to the height of the div?