This case usually happens because your sofa is moved against the wall that is awkwardly blank or because the space is simple just to small that nothing ever fits there. You may opt to leave that space at is. Although, there are better ways you can make use of that space. Here are some examples how:
One traditional way of fixing your problem is this. Install a shelf on the wall behind your couch. It is a fantastic way to maximize that space and at the same time the books adds elegance to your living room.
This is another way you can do in a narrow space behind your sofa. You can sandwich a very narrow table between the sofa and the wall. You can make use of this table to put any decoration that you prefer such as plants, lamps and frames.