First, it's important to know that the big bang wasn't an explosion of matter into empty space—it was the rapid expansion of space itself. This means that every single point in the universe appears to be at the center. Think of the universe as an empty balloon with dots on it. Those dots represent clusters of galaxies. As the balloon inflates, every dot moves farther away from every other dot. The space between clusters of galaxies expands, like the rest of the universe, at an accelerating rate. (Gravity keeps the clusters themselves the same size.)