Its confusing to think that the Universe is 78 billion light years across considering that we think its only 13.7 billion light years old. So I tried to construct an analogy that I could tell to kids. Can you tell me if this analogy is ok? I'm in the back of a pickup truck facing backwards and you are 5 miles behind me standing still. My truck is moving away from you with every second that passes by, so after a while we are 6 miles, then 7 miles, 8 miles, 20 miles apart, etc. Think of my pickup truck as the edge of the Universe and you standing still, as the Earth. I have a gun aimed at you 20 miles away. As I fire the gun, the bullet takes a while to travel the 20 mile distance between us before it hits you. But when you feel the bullet hit you, my truck is now much further away than 20 miles because the whole time the bullet was travelling towards you, my truck was travelling away from you. Think of the bullet as light. So this means that even though here on Earth we see light that is 13.7 billion years old (really more like 13.4 billion years old, but we don't need to get into it), this light has been travelling towards us for 13.7 billion years, while AT THE SAME TIME, the Universe's fringes have been expanding away from us for 13.7 billion years. Well then, we would expect the Universe to have a radius of 13.7 billion + 13.7 billion years = 27.4 billion years. Multiply this by two to get the Universe's diameter of 54.8 billion years. 'BUT WAIT!' you say. I thought the Universe's diameter was 78 billion years across? Well, the Universe WOULD be 54.8 billion years in diameter, EXCEPT that its expansion rate is accelerating. AKA. The truck is accelerating faster and faster away from you. Astronomers have seen light from Quasars that are 13.4 billion light years away. The crazy thing is that the fringes of the Universe are expanding at a speed faster than the speed of light (its important to know that the fabric of the Universe itself can expand at speeds greater than light speed). THIS MEANS that every year Quasars we could see last year, suddenly blip out into darkness, WE CANNOT SEE THEM ANYMORE because they are receding from us faster than the speed of the light that they send at us. Just imagine if the truck sped away from you faster than the bullet I shot towards you, the bullet would never hit you!