Alright, I was gonna post this a while ago, but kept forgetting, so I'll do this now before I forget. ...Again.
Alright, so, about 3 months ago, I left a witchcraft group on facebook. But, before that, there was a massive 'fight' or debate on Christians and religion. ..I started the debate hehehehe :P... Anyways, multiple members and admins were debating in the chat. At some point, it got too intense and the post has been deleted because it made people frustrated and started breaking the rules of the group and blah blah blah. And well, before I left, I got a message from one of the admins of the group and said
"Hey there, I hope you don't mind that I deleted your post. It was getting out of control. By the way, while in the debate, you intrigued me and so I read your soul, I hope you don't mind. Now, I've been willing to ask, how can a girl so young, have so much darkness in her?"
It was rather confusing. I didn't know what he meant by that. I didn't feel depressed. I am perfectly happy with my current life. I am not a bad person, or 'evil' whatever. So, I'm wondering, is there something else he could have meant by that? Or do you think he could have simply just been wrong about me. He has even given me the nickname of "Little Dark One" xD.
So on to your question. Frankly , there are many possibilities.
But first , let me address the idea of Darkness.
Concept of Darkness
Darkness is not all bad. It is not all negative , though it can be negative at times. It is not always synonymous with depression , evilness or cruelty. It is not always a place that something nasty festers nor is it always bone rattlingly cold.
From what you described , that is only a part of the aspect of Darkness.
It can be at times fairly pleasant , surrounding you like a cool and comforting cloak.
The idea of "Dark" and of "Light" is frankly very hard for me to put into words for you. Remember , that there always is a balance and is not all negative or positive. I rambled abit here but I hope this short rambling gave you some thought.
Possibilities
As you said , there is a decent chance that he is wrong about you and his reading
Like DarkDruid mentioned , a past life could have left it's mark on your soul. Some Chinese believes that even though their memories are wiped clean with each incarnation , but markings , non-tangible wisdom and a being's spiritual grace are left on the soul. In fact , many systems and faiths have similar opinions on this.
Take note that your soul is not your only body. You still have your mental body and physical body etc. And remember that while experiences eventually flow down eventually , they do not always compute.
Take note the account of lingering energies and traces. It can affect a reading. Were your mood a few hours ago or a few days ago particularly dark? Remember , Darkness need not be negative as I mentioned above!
Also , there is always the possibility that his reading totally failed.
If this person read you, why don't you just ask them what they mean? Readings are subjective to the reader's perspective. What dark means to them actually matters and is your exact explanation. What it means to us won't apply.
Well this is all perception. You perceive yourself as good, yet you laughed at the heated debate you caused [according to your first post] and while it is possible to read someone's energy over distances [I have a few online friends who do it] this admin seems to of been upset and wanted to say something hurtful to you. [Otherwise why be vague?]
They didn't mean you were evil or had a dark soul, just that you have a lot of darkness within you. This isn't really a bad thing [we all have darkness and light within us] but it seems it is out of balance is all. You may not be depressed, you may not curse others or go around kicking puppies, but it sounds as though you find it amusing how much trouble you caused and how people don't like you because of it. If this is normal and you delight in how others react it could simply be youth [I remember similar situations with friends in my teen years] but it could be the beginning of worse things to come. I would suggest you reflect and take a good look at yourself. No one is perfect but you might find flaws you wish to change. Understand why you do certain 'dark' things and try to break this habit should you conclude it is someone you don't want to be.