Apple stopped Thunderbolt display and Cinema display line.
Recently announced an ungodly expensive 'reference monitor'
beyond the need and budget of most people.
For my iMac, I bought two Apple-recommended 4K Dell monitors to go with my retina iMac. The Dell 4K monitors work fine.
But Apple dumped its monitor business without creating a way to adjust 3rd party monitor brightness from the keyboard. That's unfortunate since there's an industry standard DDC protocol to make it possible to control brightness 'over-the-wire'.
Adjusting Dell monitor brightness from the physical panel button/menu is a hassle. It's tedius, multi-step, awkward and worse if you have more than one external monitor.
I haven't found any good apps that resolve this. There's one on the App Store I found that looks like it works well, but it's badly implemented. It simply inserts an overlay graphic with an opacity setting so it darkens the screen by covering it.
The problem is it darkens screenshots, ruining them!
The apps I tried are flaky, badly implemented, limited, badly designed…
Best Answer
I've found Lunar.app and it syncs the brightness from your PC to the external monitors. It has a lot of other features that I not yet have played with, but a uniform brightness is already great. The app looks nice, so thats a plus (and it's free).
Here is an article on lifehacker.com, because the information on the Lunar site is really sparse.