Rich People Need It, Poor People Have It

My daughter riddled me this,
One in a chain of riddles
She’s recently memorized,
And the answer is nothing.

A peal of laughter,
Followed by a cough,
The ghost of influenza.
She’s getting better.

She has no idea
How obsessively I’ve thought
About this riddle’s answer,

The hinge of the universe
Enabling all to be real
That is itself nowhere real,

No subset except itself.
Great theories lose it.
Poor theories grasp it.