Why is the top half of my screeen darker?

I just got a new iPhone 6 about a month ago. It hasn’t been used before. It just started this weird thing where the top part of my screen is darker than the bottom part. It’s a little bit more than half. These pictures show what it looks like:

The first picture is one taken with another iPhone, and the second one is from a screen recording.

If I change the brightness level, both parts change, but the top is still darker than the bottom. If I turn it off or take a screenshot, everything is normal. If I restart my iPhone, it goes back to normal, but when I go to watch YouTube, it changes back to this. I can watch YouTube on any other device and this doesn’t happen. If I record my screen, the video shows the darker part. I don’t know what the problem is.