J.R.R. Tolkien reads from 'The Hobbit: Riddles in the Dark'

It cannot be seen, cannot be felt,
Cannot be heard, cannot be smelt.
It lies behind stars and under hills,
And empty holes it fills.
It comes first and follows after,
Ends life, kills laughter.

One of Gollum's riddles for Bilbo. The answer is "dark".