After I migrated from f37 to f38 my screen became brighter than usual even though brightness level is same. Black pixels are no longer black they are rather gray, they are black but they emit like some kind of white brightness which makes them gray. ( so whole screen looks like it’s emitting white light )
You need to provide enough information for us to understand the problem. Is your system a laptop or all-in-one? What model? Is your screen the builtin display? Do you have an OLED display? Do laptop brightness keys have any effect? Are you using Fedora 38 Workstation, Wayland or Xorg, or something else. Does the Gnome Settings brightness control have any effect? Have you applied all Fedora and vendor “BIOS” updates?
From your description, and assuming a non-OLED display, the problem appears to be with the backlight level. As is often the case, Arch linus has a good article for Linux Backlight. If you can determein the appropriate backligh control
methods for your system, that could help you search with journalctl for relevant error messages.
Some systems control brightness using DDC/CI, including the iMac I’m using now. The article tells you how to determine if your system supports DDC/CI. I have used DDC/CI from the command line when the Gnome Settings wasn’t working (it was fixed with an update).
It’s laptop with latest updates including firmware. It’s GDM. Gnome brightness does work, but as explained it’s kind of giving that white light. It’s LCD display. Brightness keys are working, they are basically controlling gnome settings brightness.
Many systems adjust backlight based on the background illumination, using the camera. The problem could be camera signal being interpreted as a very bright illumination. Is camera working normally?
Sometimes settings are stored under the user home directory and could be corrupt. Try creating a new user to see if the problem affects a freshly created user configuration.
Update, I’ve always had “Screen Blank” option on 2minutes, after 2 minutes screen turns off but whenever I do some kind of action it turns back on immediately, it doesn’t work now, screen won’t turn on anymore I have to shutdown my laptop.
No, my camera lid is closed alaways whenever I’m not using camera, so I don’t think so 
If the camera is part of the issue this may be affecting the outcome. The camera sees black instead of an actual room light level and the backlight tries to illuminate maximum (exactly opposite of what would be expected)
It was also closed on f37, but it worked just fine.
You can’t assume things are implemented the same way in F38 as they were in F37,
particularly anything that affects power consumption.
Backlight controls went away for me in early F38, but ddcutil worked from the command line until GUI control came back. Many of the linux issues I have encountered over the years were due to new features that broke old code but only affected a small category hardware so linux developers were unaware of the issue. In most cases, another user had provided a bug report with patch by the time I discovered the issue. It does. however, take time for such patches to be verified and backported to distro kernels.