Word That Starts a Composition

__________ is missing. I cannot find it.
I look and look, but it is lost.
Without it, I am hollow
Lacking a vital organ,
That which is important to my soul.
It cannot function in a way
Cannot function in that way
Which it was born to
and so, I am lost, too
Looking for that which I cannot find
Floating on nothing
But focusing on all things
And still I cannot find us.
It is lost to that monstrosity
That void from which nothing can run
And so I am lost to it, too.
bUt us lost souls turn into found souls
As infinity grows short
And I am dying during my growth
I am always dying
Growing similar to a lack of my body
Whilst I grow into it.
And still that which is lost
I cannot find.
In hindsight, I could look again
But that is difficult
And difficult is coming back
To look for what I could not
And which is now a job for
who follows my lost soul.
_______ is missing. I still cannot find it.