It's a meditative poem by Mark Strand.
These lines stood out:
Wherever I am
I am what is missing.
(There are many ways of looking at this, including the more depressing one of "I'm just a space-filler", a collection of atoms interchangeable with those of, say, a warthog, which could also fill in what is missing, as could an intestinal worm or a mudsucker. But I don't get that feeling from this poem. I get the feeling of completion, that we're in this world to help complete things, and what that means may vary from one person to another.)