system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can you be in a pitch black room, close your eyes, and see grey?

Why is the blackness of my room darker when my eyes are open than when they are closed? When falling asleep last night in my pitch black room, I noticed that when I closed my eyes there was a grey-ness, instead of the same black that I saw when my eyes were open. There were no prominent light sources in my room and the windows were entirely blacked out. Please explain!
gpt
I can't specifically answer your question, but there's a phenomenon called phosphene which describes how our visual system doesn't stop working even when denied light. 

I would fathom a guess that when your eyes are open in a pitch black room, it may not be as 'pitch' as you think, allowing barely detectable light to stimulate your eyes. When you close your eyes, effectively blocking light and creating a truer 'pitch black', resulting in the phosphene phenomenon to occur. This might be why it appears 'brighter'.

But that's just a completely uneducated guess.

EDIT: Forgot to add that phosphene is the little lights and flashes of colour when you close your eyes. Same as when you rub your eyes, etc.

Also typo.