The shadow self is a concept that one needs to confront throughout our entire life, unless we want to risk being ruled by it. There are so many levels to it. Some of which can be understood through scientific inquiry and insight, while others are only available by observing our subjective experience. You might find this a good exploration of the shadow — https://www.psychologytoday.com/blog/evil-deeds/201204/essential-secrets-psychotherapy-what-is-the-shadow