Hardest modern concept to explain?

[Romulator]

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?

It's funny. I actually suspect other planets to have started communicating with us... or at least, other planets amongst others by now. In the least, we could not be alone if that were to happen.

I can imagine, someone gets like some paper or something similar, writes a message that says "Hello, I am from Troebnsk, a planet in the Yuajn quadrant, and I would just like to say, you're not alone." but would be in some weird symbols even we could not possibly translate with our diverse collection of languages and communication, sends it to NASA, and they get this weird suspicion it is in fact from out of space.

I mean, it would be cool to at least be confirmed as not the only thing life form.

Another tough concept to explain might be the movie Back to the Future.