Death has always been portrayed to be as old as God (may be older - season 5) and equally powerful to God (according to Death it may be more powerful as it can kill God as well). So how come the Darkness has never heard of Death? In terms of existence earlier there was only Darkness but then God and Death came into existence and both are equally strong. But when Dean mentioned about Death (Season 11 episode 1) to Darkness, She said she hasn't heard of Death (possible because being trapped since before other things came into existence). How come she hasn't heard of Death when at that time only few creations existed?
Assuming this isn't just an inconsistency, I can think of a few possibilities. There's no definitive explanation (as yet.)
- Since Death supposedly pre-existed the archangels, I think we can get away with saying Death simply never interacted with the Darkness, and God and the archangels didn't mention him when battling the Darkness and sealing it away. So the Darkness never knew that Death existed, but God filled in Death (or he simply observed what happened.)
- Characters lied to us. Death didn't actually precede the archangels, and was in fact created by God afterwards. Maybe not necessarily a lie, maybe Death is so old he just forgot. [I think this is worth considering since he claimed he was going to reap God one day, and that now seems somewhat improbable.)
- Our whole narrative of what happened with the Darkness is wrong. After all, apparently nobody expected the Darkness to be a woman.
Death said that he and God can not remember who is older. But I think only Death can´t remember, because God is older. God created the world out of nothing, which means the Darkness was the first existing being. After the Darkness God was created and when God created Death, the Darkness was already locked away. Maybe that is the reason why Amara said, she doesn't know Death. Remember, Chuck has never said, that he can´t remember, who is older, him or Death. I think Chuck exactly knows, that he is older than Death and Death was created by God, after Amara was locked away. Maybe Death forgot about that or God changed his memory, so that he thinks both of them are the same age
She said that she doesn't know this death and he doesn't know her. My guess is that she can't be killed. She doesn't know what death is because no one died before she was sealed. God needed to seal her. At the very least Death should be as old as the first killable thing. The Arch Angels. If God and the Darkness are unkillable(brother and sister as we now know) Death had no reason to exist/interact with them. He assumed he might reap God one day but was in no rush to. Maybe he couldn't if he tried. If Death existed at the beginning with God and The Darkness maybe he didn't know either one of them. He might've met God after she was sealed and God told him about her. Or maybe Death is her son. And she betrayed her so she's basically disowning him. Just some theory's
Death is confirmed as a being equal to God in strength and nearly equal to him in age. It was confirmed that Death predates the Archangels as he was able to effortlessly enter Lucifer's Cage and then leave, when the Cage is capable of holding the two most powerful Archangels: Michael and Lucifer. Death could be slightly younger, the same age, or older than God. I theorized Death simply hid himself, much like God and Amara could from each other, during God's war with his sister and merely witnessed it, as he possesed much more information on Amara than the angels or demons. Amara is the only being Death truly feared, which is probably why he hid himself.