There's a certain subtlety to the main actor's performance as his character goes through life pretending everything is o.k., saying he's fine when really he is not. It's the sort of thing you will only pick up on if you've been through it yourself.
He's trying to keep it together for his kids, but he doesn't know quite how to proceed with his life now that his wife has left him and so much has changed. That's where the title comes from. He has "empty spaces" in his daily routine because he has forgotten how to live without his partner.
But everything is not o.k. like he wants it to be and things gradually begin to deteriorate, leading to a psychotic episode involving a garbage disposal. Other than that, he only verbally hints once or twice at the fact that he is suffering inside. Men don't really like to talk about things like that, and this one is no exception. Instead he does what so many of us have done in his situation: he drinks and he drinks.
There's no big climax or clear resolution here and the future for these characters is left open-ended and somewhat ambiguous, as is so often the case in real life. The audience isn't left with hope or a happy ending or anything like that, or maybe they are depending on what they choose to take away from it. I suggest you watch it twice, because the second time around you just might pick up on a few things you missed the first time.
