What Am I Riddles

Question: I cannot be seen, cannot be felt,
Cannot be heard, cannot be smelt.
I lie behind stars and under hills,
And empty holes I fill.
I come first and follow after,
End life, kill laughter.

What am I?

Question: There is a story that a man and not a man
Saw and did not see a bird and not a bird
Perched on a branch and not a branch
And hit him and did not hit him with a rock and not a rock.

[How is this possible?]

Question: Something different to show every week, but I'm still the same place.
If you're eating it's probably butter that you taste.

What am I?

Question: We have what we did not find; what we did find we left behind.

[What do we have?]

Question: Alone I am 24th, with a friend I am 20. Another friend and I am unclean.What am I?