The "Big Bang" is commonly described as an explosion, but this is not really a good analogy. In fact, the universe was created through an expansion of space.
The Big Bang
In the big bang theory, the universe formed as a primeval fireball about 15 (or so) billion years ago. Initially the entire universe was compressed to the size of an infinitesimal point. At the moment of formation this primeval fireball began expanding and has been expanding ever since.
The initial expansion of this primeval fireball is almost universally described as a huge explosion. An explosion however is not the best analogy to describe this expansion. Why? In an explosion, such as a bomb or firecracker, matter blasts out from a central location and fills up space that is already there. The pieces of matter are moving away from each other because the blast gave them a large velocity away from the center of the blast.
Expansion but not Explosion
This is not what happened in the big bang. It is true that distant galaxies all appear to be moving away from us. That is not because we are the central point in the universe. It is also not because an explosion gave them a large outward velocity.
Galaxies are moving away from us because the universe is expanding, as predicted by general relativity. That means that the space between galaxies is expanding. In the initial big bang the universe started expanding. Matter and energy already filled the entire point sized universe. Matter began moving farther apart because the space between the matter was expanding not because it was rushing out to fill space that was already there.
Raisin Bread Analogy
Making a loaf of raisin bread provides a better analogy to the big bang and the expansion of the universe than an explosion. When making raisin bread a baker mixes the dough (with lots of yeast), mixes in the raisins, and lets it rise. While the dough is rising the raisins will be moving away from each other. This motion does not occur because a clump of raisins has exploded and blasted the raisins out to fill up the already expanded dough. Rather it occurs because the raisins are already spread through out the dough and the dough is expanding. In the big bang, space itself is expanding just like the rising dough for raisin bread.
Another analogy to the big bang and expansion of the universe is blowing up a balloon. Draw some spots on the balloon to represent the galaxies. While blowing up the balloon, the spots will move farther from each other, just as the galaxies are moving farther apart in the expanding universe. Just as the balloon has no center spot, there is no center galaxy in the universe that we need to be in to see the expanding universe.
An explosion did not fill the universe with matter, rather the universe itself began expanding. Rather than thinking of the big bang as a giant explosion, compare it to rising raisin bread dough or blowing up a balloon.
Further Reading
Barrow, J.H. and Silk, J., The Left Hand of Creation, Oxford, 1983.
Silk, J., The Big Bang, Times Books, 2000.
Harrison, E.R., Cosmology The Science Of the Universe, Cambridge, 1981.
