I've had the phone for a few months, and I've always had dark colour scheme on, so I'm not sure when this occurred. Could this be dirt under my screen protector, wouldn't that definitely show a bubble?
If you have a screen protector there is a good chance that is what it is. When you turn the screen off can you see this particle in the light from your room?
It's highly unlikely that dust got under the screen and then migrated all the way to the center of your screen.
I just removed my screen protector and it doesn't come off. It is definitely a dead pixel.
Well that's unfortunate.
So much for my theory. ;)
I enlarged the picture to the maximum and at maximum magnification you can see that it is not a dead pixel but some irregular dirt, even if under the glass cover of the display, but it is dirt. Dirt had to get there as soon as the phone was assembled in the factory.
In the picture it looks like dirt, because it doesn't look square. IRL it is a square dot. I never noticed this before.
Regardless if it's dirt or dead pixel(s), you have to take it with ASUS Support.