If you installed tlp, you can set the default brighteness there. If not, check your battery settings, since the default power saving configuration may drop your display brightness to the lowest setting to conserve power (its one of the largest power draws on any system). Checking the video card settings may reveal that as well.... unfortunately, it is controlled by many parts of the system.
You would have to look at the advanced portion, under display. It breaks down all the devices, from usb's and hard drives to specific hardware devices...cpu, gpu, wifi/ nic.... it is listed and adjustable.
Open the file /etc/default/grub using gedit or any other text editor.
Find the below line.
Find the line GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
Change the line to GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi_backlight=vendor"
If that one did not work and you have a nvidia card you might try this one too.
sudo gedit /etc/X11/xorg.conf.d/10-evdev.conf
Add the below lines entirely at the end of this file and save the file. Section "Device" Identifier "Device0" Driver "nvidia" VendorName "NVIDIA Corporation" Option "RegistryDwords" "EnableBrightnessControl=1" EndSection
Once you are done, restart the system.
You should be able to control the brightness now using the laptop hardware keys.