These theories are accepted because they absolutely make perfect sense if you understand -just a little bit- of how they actually work and there is plenty of observable evidence to support them.

The Big Bang occurred 13.7 billion years ago. At its first point it was compacted into what we call a singularity. Which is a point of zero volume, but very high mass, which makes the density infinite. There was not yet any matter. However, there was a lot of energy in the form of radiation. (We know singularities exist, because we have mathematical proof they lie in the center of black holes). All that causes the Universe’s expansion is the total amount of its energy density. Radiation — things like light — not only dilute like matter does, it gets redshifted, which means its energy density drops faster than matter does. We know that energy can be converted into matter, and that is exactly what happened when radiation started producing matter just moments after the big bang. As the Universe cooled, matter became more important, and dominated the expansion rate for billions of years. If you want to know how things like atoms such as hydrogen were created and where the stars came from, I can explain it to you but that is besides my current point: The energy density of the Universe is, in other words, the total amount of matter, radiation, neutrinos, and all other forms of energy in the entire Universe divided by the Universe’s volume. And that equation is all that determines the rate of the universe's expansion.


As for what happened before the singularity, Think of the universe as a giant balloon with billions of dots on it. The material that makes up the balloon is "Spacetime", which we know for a fact makes up the entire universe. Space and time are directly related. The big bang is the theory that explains how the balloon, or the universe, expanded from a singularity. So, because of the nature of space time, the closer we get to the point where the universe (or the balloon) was a singularity, the more time slows down. At point of when it was a singularity, time is infinitely slow, or basically non existent. Now, let me ask you, how can there be a point before the big bang if there is no time, or space time for that matter?

I'm not sure where you got the antimatter collision from, because that has nothing to do with the expansion of space time itself. Just how matter got here.