The origins of the universe are a mystery, but by understanding the scientific laws about gravity, mass, energy and thermodynamics, you can speculate better.
First, let me say that while there is no scientific proof of what happened before the big bang, or anything else to do with the origins of the universe, these scientific laws are facts, not opinion.
In this article, I hope to give the average person a basic understanding of the way the universe works, and the theories that exist about the origins of the universe, so they can make their own mind up.
The Laws of Thermodynamics
The first law of thermodynamics says that energy cannot be created or destroyed, it only changes form. This is a very simple law to understand. If you burn a match, it turns into ash, smoke, and carbon monoxide, it never disappears. The same is true of the eternal energy that makes up the universe.
The second law of thermodynamics says that all things in an isolated system will gain entropy, which means cool down, slow down, fall apart, etc, unless something causes them not to. This law is sometimes misinterpreted when the last part is forgotten.
When you heat a glass of water, the molecules move faster, then they cool down again. Heat can only be transferred from a hotter source to a colder one.
The third law of thermodynamics says that when the temperature approaches absolute zero, so does the entropy. At absolute zero, the electron stops revolving around the nucleus and collapses, taking away 99.999% of the volume of the atom, but not the mass.
This is what makes a black hole, and it is quite hard to do a test on a black hole, but they call it a singularity of infinite density, because it could not be more dense than it is. There is a lot of gravity in a black hole.
The law of Gravity
Newton could not have made it more simple than his famous saying "What goes up, must come down". This is about all there is to gravity, it has no limit, it just gets weaker the further away you are from the mass causing the gravity.
Gravity is a bending of the fabric of space/time, and if you like, you could imagine it in two dimensions with lots of heavy balls pressing down on a trampoline.
With no source of propulsion, and no gravity existing on the other side, the universe will surely stop expanding, and begin to contract because of the gravity of itself.
An object can only continue to travel at the same speed through space unless it has a source of propulsion. There may be propulsion from the stars which is pushing the expansion, but eventually they will burn out, and form black holes.
Given no source of propulsion, and the gravity of the entire universe to pull it back, the universe cannot continue to expand forever, and the expansion must slow, until it stops, and then starts a big crunch. This is a fact, based on these simple, proven scientific laws that anyone can understand.
Einstein's Laws of Energy
Einstein is most famous for his equation E=MC2. What this means is that in each kilo of mass, there is a kilojoule multiplied by the speed of light squared of energy.
The same is true in reverse. Mass equals energy divided by the speed of light squared. This is one of his less famous equations. The first one led to the development of the atomic bomb, and perhaps one day the last one will lead to teleporting technology or some such thing.
Anyway, as the first law of thermodynamics says, energy cannot be created or destroyed, what goes up must come down, and when it comes down, mass is converted to energy in a big bang. Big bang, big crunch, big bang.
Why is this not considered a fact by some people? Because it cannot be proven one way or another in an observable scientific experiment. I will briefly tell you about the two most popular big bang theories.
Big Bang Theories
There are two main theories on the big bang, the big freeze theory, and the many worlds theory. In the big freeze theory, the universe and time both began at the big bang, and there was no before.
This theory suggests that the universe existed as a singularity of infinite density in which time did not exist, and for no reason at all (you have to guess the reason), it just began to expand.
In the end, this theory suggests that the universe will continue to expand away from the gravity of itself, until the black holes begin to fall apart, due to proton decay, and all that will be left behind is mass-less particles.
There is more than one thing in what I just said that doesn't seem to make any sense, based on the scientific laws I just told you about, and it doesn't even give a cause for the big bang, but nevertheless, this is the most popular big bang theory today.
The many worlds theory suggests that the big bang has happened before, and will happen again, perhaps an infinite amount of times. It succeeds in explaining the cause of the big bang: the collapse of the previous universe in a big crunch.
At the time it was discovered that the universe was expanding from a big bang, it was taken as proof by most of the scientific community that God created the universe just like in Genesis. Perhaps this is the real reason for the popularity of the first theory.
I hope that my explanation of these simple scientific laws gives you a better understanding of the origins of the universe, so that you can make up your own mind what to believe, but it is still a matter of belief, as no one big bang theory has been proven.