If you're on Dark, you probably want to listen the amazing voice of Nena again and again.
The music is "Irgendwie, Irgendwo, Irgendwann" by artist Nena. It was released in 1984 on the album Feuer Und Flamme.
Here is my favourite version of it:
And here is the original clip they used during the series:
Youtube removed the first link so I added another one. For now it works.