Seems like you can barely pick up a newspaper or read news on the internet without someone claiming that established science is "controversial." More on that tomorrow. But first, we need to understand the levels of disagreement in science. To do that, let's talk about the Big Bang.
The Big Bang is the theory that the Universe originated from an incredibly hot, dense, and energetic "explosion" about 13 billion years ago. All the matter in the Universe today was compressed into a region much smaller than the size of an atom way back then. Since then, the Universe has been expanding and cooling, with the matter combining in different ways to make the galaxies, stars, planets, and so on that we see today.
Although the Big Bang is referred to as a "theory," it is as close to fact as one can get. There are well over 10,000 professional astronomers in the world, and all but a few (maybe 10-20? I can name four) believe that the Big Bang Theory is an accurate description of the Universe from about 10^-32 seconds (one hundred-millionth of one trillionth of one trillionth of a second!) to the present. So is the Big Bang Theory controversial? No! It's barely even debated.
Just before the Big Bang, many astronomers believe was a period of "inflation," when the Universe expanded much, much faster than the speed of light. This period would have only lasted 10^-32 seconds, and the Universe went from much smaller than an atomic nucleus to the size of a grapefruit. It doesn't sound like much, but the Universe grew in size by one hundred QUINDECILLION times (a one with fifty zeroes after it). Now, inflation is not a proven theory. Most astronomers believe that some form of inflation occurred, but we disagree on what form that inflation took. So, we can say that the idea of inflation is mildly controversial, but the exact details of that theory are fairly controversial.
Finally, we come to what happened before inflation. This is highly controversial -- the few theories that exist are fairly hand-wavey, and it is not yet possible to test these theories. Did the Universe come from some frothy quantum bubble in a larger-dimensional space? Maybe the Universe started when two branes of spacetime collided? Maybe this is where a Creator steps in? We really don't know.
So, in summary, the Big Bang is non-controversial, the theory of inflation is mildly controversial, and hypotheses about the pre-inflation universe are highly controversial.
Tomorrow, we'll look at science that is a bit more relevant to our everyday lives.