Commentary for Nedarim 78:8

Seven items were created before the world was created -- meaning, that they entered the (God's) thought process to be created before the world was created, because the world can't stand without them. As it is stated elsewhere, that the thought of the people of Israel preceded all (in God's thoughts). And it is self-evident that the world was not created without there being a Gehinom for the wicked (an eventual place of judgement for their wickedness in this physical lifetime).