Why is my Samsung screen dark?

I have a problem with my phone, I think it's a Samsung Galaxy SII Plus if that is of any relevance. The screen has been darkened for the past months. All I can find when I google it is people whose screens go black, but that's not what happens. It's more like the brightness is taken down a little, but I have it on full so that's not it. The contrast also seems slightly increased or something, because I can't see details in dark pictures. The strangest thing is that when I pull down the menu thingy on top of the screen (where you can see notifications, activate wi-fi etc) a little bit, details appear in dark pictures even though it makes the screen a little darker! I've tried adjusting brightness but it doesn't work, I've tried activating and then deactivating auto brightness, but that doesn't work either. When I restart my phone, it goes back to normal brightness for a while but then pops back into darkness after a few seconds or minutes. It hasn't always been like this, but it has for a very long time now. What's wrong, and how can I fix it?

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