If you don't like science, I'm going to have to warn you that this essay probably won't be all that exciting for you. On the other hand, if you don't like science, or don't "believe" in science, then you're probably the one who needs to read this the most. The reason I say that is I have yet to meet someone who truly understands the Big Bang theory who disagrees with it. However, I've met plenty of people who disagree with what they think is the Big Bang theory. Thus, I intend to lay out what the theory really is, and what proof we have of it. Then, once you're informed, feel free to deny the Big Bang as much as you like. Are you ready?

I'll start by explaining what the Big Bang theory is not, as many people have some pretty serious misconceptions about it. First, and contrary to the title of this section, the Big Bang is not actually the origin of the universe. Rather, the Big Bang theory describes how the universe evolved immediately after its formation and the subsequent changes, but it does not describe how the universe came to be. In fact, while we have some theories that I will get into later, we really don't know how the universe formed, as the two great theories of physics, relativity, which governs the very large, and quantum mechanics, which governs the very small, don't play well together. In other words, when the two meet, as in the Big Bang or in black holes, the theories break down. In fact, the merging of these two wildly successful theories into a cohesive whole has been the goal of physics since Einstein. But that’s a topic for another time. For now, remember this: The Big Bang does not describe the origin of the universe.

Second, the Big Bang theory does not predict that the universe was ever point-like. We can extrapolate back to that point, but such an extrapolation is probably wrong because, as I said before, our current theories break down in such conditions.

Last, the universe did not "explode" into already existing space. What did it "explode" into then? We have some ideas, but we don't really know. Perhaps this question makes no logical sense as anything the universe could have expanded into is, by definition, part of the universe. However, there is no question that the universe has expanded and will continue to expand.

What, then, is the Big Bang theory?

Essentially, it says that the universe used to be very small, hot, and dense, and since then it has expanded and become cooler. We can imagine this expansion by way of analogy. Pretend that the universe is a two dimensional drawing on the surface of a balloon. The expansion of the universe is like the blowing up of the balloon. Note that space itself (the surface of the balloon) is expanding and that every point on the balloon is moving away from every other point. This movement is not due to motion through space; rather it is motion by space.

This provides the first test of Big Bang theory. And, in fact, the universe is expanding. We know this because light coming from distant stars is red-shifted. In other words, the light waves are "stretched out" due to the stars moving away from us in all directions. This is similar to the Doppler effect, which occurs with sound waves. This is where the pitch (wavelength) of sound waves changes as a vehicle or something else moves past us.

Further proof of the Big Bang theory comes from the large scale homogeneity ("smoothness") of the universe. This is a requirement for Big Bang theory, and while it is clearly not true on small scales, over millions of light-years the universe is very uniform.

A third piece of evidence comes from the existence of cosmic microwave background radiation (CMBR). Essentially this is leftover electromagnetic radiation (light) from when the universe first cooled down enough for atoms to form. When this occurred photons (light particles) could suddenly travel large distances without coming into contact with matter. Because the universe has expanded a lot since that time, the wavelengths of these photons are really stretched out. Because the universe was relatively small at the point in time any irregularities would also be small. So if we were to detect CMBR, it would be very uniform. There are also a couple of other properties of this radiation that are to be expected, but they are quite technical and not worth getting into.

So does the CMBR exist? Yes. We have detected uniform radiation as predicted by Big Bang theory. In fact, when it was first detected by a supersensitive radio antenna as static. The antenna operators were very confused by the signal, even going so far as to permanently scare off all the birds around the device, whose poop they thought was interfering with the signal. However, the static turned out to be CMBR, as first predicted by a paper around the same time.

There is a multitude of other evidence, but this is a basic sampling that should establish the high degree of certainty in the Big Bang theory.