Remember I said I don't know that much? Anyway, karma, for those who are not sure is a cause and effect relationship. If I do well by others then hopefully well will be done back at me or so we hope. This is how I see it. I can be an ass to others and have no cost for it come back to me. That is a possibility, however it hasn't been my reality. Usually always when I have done a harm or a selfish act, I get back some equivalent of it. It is why I subscribe to the cause and effect theory. My own experiences teach me that.
However I am but one person on who I rely to convince me that karma is real. I have heard others tell the same type of stories but that is mostly anecdotal. So I only subscribe karma to myself as I know it works for me. It makes sense otherwise since I am a good person who likes to do good things. Even when bad things happen back at me I don't blame doing good, I just recognize that some things need to happen and that is what they are doing, for whatever unknown reason to me. Again, my ignorance of many things is still overcome because I know that my doing good doesn't cause me having bad done back to me. It would happen probably more often if I wasn't doing good. This all may be a bit confusing but to me it somehow makes logical sense.