Hardest modern concept to explain?

[FlawlessHappiness]

So far, all of the things should be ok-easy to understand and explain.
It would just be a shock to the person, yes, but everything is build with logic. So if the person is willing to try to understand it, it shouldn't be a problem.

I think the question got confused with "hardest modern concept to explain WHY".
Why, is a completely other question Here, twerking and such would be relevant.

I can't think about anything that someone with enough knowledge wouldn't be able to explain.

The thing about astronomy is that you could create a movie that explains it graphically, and bam, then it's explained.