Bottomless Eternity

It burns with agony never ceasing Constant always ever increasing It never stops it never tires It destroys all yoursecret desires Its intencity you will always feel Claws dig deep before you know the deal An endless abyss unbelievably deep Wide its mouth an impossible leap Longing its core hard to kill the heart The only way to stop it is to fight from the start In the end there are only two answers To fight the infection or end in disaster It is a great battle a very long fight The dark rages against the light In the end it is just a brutal melee The dark never allows for the battle to be easy Another one of my older poems this piece has gone through much editing. It is also a riddle. Can you guess the thing of which I speak? One hint, it is not material.