Why is my skin full of dark and light spots, along with uneven skin tones?

I know partially it can be caused by hormonal fluctuations from puberty, changes in diet, and also exposure to the sun. But I have it even in places not exposed to the sun and I my diet isn't extremely unhealthy. I do eat junk food a few times a week but my skin was like this even in highschool when I did eat healthy.
Why is my skin full of dark and light spots, along with uneven skin tones?
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