For many years, the beginning of the universe has been called the Big Bang, but to physicists and cosmologists this isn't a good way to describe what happened. In fact, the term itself was originally meant as a derisive joke when the theory was young and still largely not believed as credible. Through the twentieth century, however, as universal expansion became clearly self-evident, the term stuck in public media and discourse, so the damage was done.
Why doesn't "Big Bang" fit, you might wonder? For several reasons, not the least of which is that it carries with it the image of an explosion going out from a center, which is also a failing of our balloon model. This error in imagery is further supported by the great irony of the actual appearance of the universe itself. While universal expansion should naturally make every point seem to be receding from every other, causing the view from any position appear to be the center of an explosion, this is compounded by the finite speed of light, which limits the ultimate distance at which stars and galaxies can be seen. Since there is a point in the past beyond which no stars had yet formed to put out visible light and because to look outward in space is to look backward in time, there is an outer 'edge' of the visible universe. To someone who doesn't understand the limitation of light speed or the idea that if everything could be seen at the same time all parts of the universe would generally look alike (called Homogeneity), the appearance of the universe is very much like that of an explosion that originated at a single place. Although this illusion may be comforting for those who believe that the earth and its inhabitants have the privilege of being at the center of the universe, it is not so.
Given the well understood laws of gravitation and celestial mechanics, at least as we currently know them, the universe would look very different if it had begun in a larger void into which it expanded like an exploding bomb. First, there should be more matter near the center than farther out, because it would have been partly shielded from the expansive forces, but there is not. Out to distances of billions of light years, the average density of matter seen is uniform to a high degree, regardless of the age and stages of growth of the galaxies. This is partly the basis of the idea of Homogeneity. Also, if the bomb idea were true, there would be no reason to expect the age-limited edge of the now visible portion of the universe to coinicide closely with what would otherwise be the actual leading boundary of material flying outward into the void. It could look just as it does, if the explosion were large enough and the visible portion were just a small part of the whole, but it would be just as likely that the outer boundary of expanding material would fall far short of the farthest possible visible distance. However, if the former were true, that the supposed exploding bomb were large enough to allow for the uniformity in what we see, then it would be far too large to have all been concentrated at one point at the instant of the Bang. There simply has not been enough time for matter to travel that far at speeds less than that of light. The latter cannot be true because what we see does indeed follow both the gradients of speed and galaxy formation that it should in order to conform to the expansion of a uniformly dense universe.
We are, therefore, seemingly left with an idea that is truly alien to our limited senses and against all rationality; that the universe is infinitely large in all directions and that all of it is expanding equally, even though we can only see a small portion of it thanks to light speed. As hard as this may be to grasp, the reality is most probably even stranger! When Albert Einstein first formulated his theory of General Relativity, he was forced to add into one of the important equations a constant, called the Cosmological Constant, to account for what he then believed to be a universe that was in a steady, non-expanding state. When the expansion was proven, he dispensed with it, but more recent developments, like evidence that the universal expansion is accelerating, has forced astronomers and astrophysicists to reconsider it. For brevity, I will leave the topic of accelerating expansion for another section, but the trouble with the Cosmological Constant led to the realization that the universe might not be infinitely large after all, yet still has no outer edge.
In Einstein's original view, the galaxies would inevitably be pulled toward one another by their mutual gravitational attraction until they all collided in a Big Crunch unless there was an outward pushing force to counteract it. But even before the expansion was first found, Einstein's equations showed that space is curved near any massive objects, in a way similar to a sheet that is depressed by a ball placed on it. Furthermore, the fact that the universe as a whole is dotted with trillions of galaxies along with its proposed nature as a continuum at the subatomic level meant that all of space must have a curvature, which is another way to describe the general gravitational attraction. He had no easy way to explain the Constant and the outward pressure it signified, though, except for one which we might recognize from our balloon model. If we tie the mouth of a balloon, the air inside it will hold it at a certain size as long as the air doesn't seep through its skin. The pressure inside it is equalized by the tension created in the skin when it is blown up. Likewise, Einstein and others said, the universe doesn't collapse in on itself because it is finite in size, although unbounded, like the skin of a balloon. As an ant on a balloon's surface can walk all the way around it and return to its starting position, so too might an astronaut in a space ship go out in a straight line in any direction and eventually return to the earth. If this is true, then the galaxies pull each other from all sides at once, so that their mutual attraction cancels out, like the static tension in the skin of the closed balloon. This part of the theory not only explained the steady state then believed by most physicists, but also the universe's inherent curvature that Einstein's theory demanded. As an ant on a large enough balloon would see his immediate environment as nearly flat, so do we see our world; yet it is curved, as subsequent observations showed.
Soon, however, astronomical observation by men like Edwin Hubble showed the expansion that undercut this model, like Einstein's theories of Relativity had done before it. As I have said, this expansion did away with the need for a Cosmological Constant, or outward pressure, to keep the galaxies from coming back together, but it did not explain away the curvature of space. Now, we had an expanding universe that was still finite in overall size. This is when the analogy of the balloon first was used in a way that began to make some real sense. No matter how much tension the skin of the balloon had, the initial 'breath' inflating it was more than enough to keep it expanding. The same can be said for the expansion of the universe. The astronomers considered that perhaps it would not expand forever, but long enough that no slowing would be seen in the visible portion, and none was. The astronomical observations before about the 1980's agreed with this theory so well that there was no need to think of another way to imagine it.
Quantum Theory came into its own in that time, but it dealt only with the very small and with forces much stronger than gravity, yet which worked only at those small levels, so the old model was unaffected by it. But astronomical instruments eventually became powerful enough to see beyond the visible edge of the universe into the time before stars and galaxies were formed, deep into the infrared range of light where they might be able to see when the universe itself existed at scales of size and energy where Quantum Theory came into play. When that happened, the model changed again, but that is a topic for next time.