We cannot define void in terms of what it is, only in terms of what it is not. Void is that which is not everything else. If everything else exists, then so must void. Void and everything else are like the Yin and the Yang. Void is not a number, it is not a char, it is not the air or the sun, it is not made up of the stuff that makes everything else, however void exists because know it. Perhaps it only exists in our minds.
Speaking of void and everything else, what is everything else really? Is it that which we see, hear, taste, experience? Or that which we can experience? We see it through a small window as it flashes past. Or perhaps we can think of a computer screen on which everything scrolls by. We cannot control the scrolling, only what we choose to witness. The void is there all along, it is where we do not see.