To Think of Nothing

At times I find myself in a state where I appear to think of nothing. Most of the people I've spoken to say that they've never experienced it, so I decided to do some digging to find out if it's possible or not. Let's start with Rene Descartes' idea of "Cogito ergo sum," which means "I think, therefore I am." If we accept that, then it should also be true that if I don't think, then I don't exist, or at least that thinking is fundamental for the human consience to exist. 

In DaMo Qigong, (here simplified) excercises can teach you how to think of nothing. By first focusing your thought of darkness, a void where nothing exists, you return to the time before your birth. Before birth, you could have no conscious thought since you have seen or heard nothing; your mind had not enough "food" to develop thoughts as we know them. Similar experiences can be achieved through meditations practiced around the globe and what's similar between these experiences are that the people who have gone through them still describe to have been in a state of hightened consciousness. How is consciousness possible withouth though?

If you sit/lay down and decide to focus your mind of nothing, then that's exactily what you're thinking of. Nothing is the opposite of something and in order to undo the thought of something in your head, you first have to be aware of the "somethingness", leading to an impossible task since undoing the something actually is a thought process. There is always a stream of consciousness rushing through your head and if you find yourself unaware of the thoughts, then they've been running on a subconsious level. My question, however, is what's really going on in your head when you find yourself in that empty state, i.e when it's not forced or something that you've consciously tried to achieve. I haven't found an answer to this so far, but my guess is that I do, in fact, think of something. A void for example, even though I'm not aware of it until when I "wake up" from the state. Maybe I even return to that time before birth without being aware of it. It's a mystery yet to be solved!