Your poem is made up of very evocative images. This gives the interpretation, as it should, a degree of freedom, thanks to the ambiguity of poetic language.
Windows, which are often a metaphor for eyes, can become darkened, and indeed they do so when our inner selves are filled with uncertainty and/or loss. And we are that space we long for as a refuge, but it is no longer there. Emptiness inhabits it, perhaps like our hearts.
Best regards, @tarazkp.
I wonder if there was a way to get an average "heart fullness" from society, what it would be over time?
I don't think that exists in a general and permanent way. Perhaps individually and at times. It is an “arduous task,” as the poet Rimbaud said.