I saw the answer for 10.10 and 10.9. Sadly I just jumped from 10.8 to 10.11 and discovered that my beautiful and perfectly well-functioning (last night) display is not kaput (too dark). Can it be made to work in 10.11 or do I have to find a copy of 10.9 or 10.10 somewhere and try to revert to that? I did backup my mac before the upgrade, but still. ARGH!!!!
Just found a simple way to do this that I already had available to me (simple for me, maybe or maybe not for others). My mac mini is set up as a triple boot (Windows, 10.6, 10.11 [was 10.8 until last night]). I have some programs that I still use that only work through 10.6 and some others times I need the newer 10.8 (and just got a newer iPhone and it needed a newer version of iTunes to allow backup which is what got me going on changing to 10.11 anyhow). Anyway, setting the brightness in 10.6 and then rebooting into 10.11 works. Obviously, not great if you have to quickly make it dimmer, but this solution is the best for me. I think there is plenty out there on how to make a dual-boot mac setup (the triple boot with windows was a tremendous headache and I would not recommend it, but I did get it to work finally).