Hardest modern concept to explain?

[Froge]

Extraterrestrial life. There is no way that we are alone in the space which hold bilions of planets, stars, and other objects.
Quick edit: maybe there is somewhere a planet which holds all of the Old Ones?

I think it would be extraordinarily rare to find extraterrestrial life that we can communicate with. Humans have evolved to be good at specifically recognizing other humans such as their facial features, but who knows what weird beings could roam other planets?