“Darken” covers a few different wishes, and the right control depends on which one you have.
Lower the whole picture
For a background behind text, a slide, or a photo that needs to sit next to darker ones, use Dim it down. It lowers everything smoothly and keeps a little contrast, so the image still reads; it works the same way when you dim a video.
Fix a picture that came out too bright
A washed-out photo needs more than less light: its highlights need pulling in and its blacks deepening. Auto fix does that from the photo’s own tones, and the overexposed photo guide explains what can and can’t be recovered.
Change the mood
Darker images feel calmer, more serious or more cinematic. Lower Gamma under More options to darken the middle tones while keeping the brightest highlights, then lower Saturation a little. That combination looks deliberate rather than underexposed.