What is 'the dark'?

What is the definition of 'the dark' or 'someone of the dark'?

I have been advised on occasions to avoid a certain person because they are 'dark'

And I read of 'dark forces' are they the same as someone 'of the dark'?

The two I have been advised to avoid (I rarely do as I am told) I see nothing wrong with them.

One does appear to have a big chip on his shoulder, the other I think has a drug problem.

Is this what is meant by 'the dark'?

Someone who has not yet realised how special they are, what they have to offer.

Someone who does not yet learnt to love?