"The Shadow sounds like kind of a scary thing on the surface, but all human beings have a Shadow. In scientific terms, the Shadow is the amalgamation of untransmuted cellular density that we carry within our bodies and energy fields. Another way to look at it is that it's the curriculum that our Souls have given us to work through in this lifetime. It's our 'stuff' that we inherit from our parents and ancestors that becomes ours to then alchemize.
One of the realities of life is that this 'stuff' gets passed down through the family lineage—both positive and negative 'stuff'. You've inherited all of the following through your lineage: physical toxins, metals, and pesticides; self-defeating and self-destructive behavior patterns and thought patterns; strengths, skills, and talents; language, dialect, and patterns of speech—and much more.
From the Soul level, it's all by design. The 'stuff' you inherit and the Shadow within that that 'stuff' becomes is the exact thing that your Soul knows you need in order to evolve. It's also a matter of karmic balance: as this is a Free Will Universe, we have the freedom to go against the Law of Love/Unity, and so we must eventually balance out all of those past-life actions that were out of alignment with Love. "