Hardest modern concept to explain?

[Froge]

don't know about you guys but I'd say it's astronomy

the greeks thought the sky was a sphere with stars "painted" on its surface, that the skies obeyed different physical laws than those on Earth, that eclipses were ill omens, etc.

It would be a huge shock to tell them about the vast distances between Earth and other solar systems, the sheer size of our galaxy, what actually causes eclipses, as well as other facts like the matter we are made up of (eg. carbon) were produced inside stars and stuff