For almost one hundred years, we have known that the universe is expanding. We have traced this expansion back in time, through to the very beginning when the universe occupied an infinitesimal point in space. This was the state of the universe at time t=0, over 13 billion years ago.
It is from this starting point that everything we are familiar with came into existence: protons, neutrons, stars, galaxies - even space and time itself are here. At time t=0, this point began an unprecedented inflation, in this instant time and space were born. This event has become known as the Big Bang.
The big bang is our current, most accepted theory for how the universe began. It was inspired by the discovery that the universe is expanding.
It was an unusual and highly counterintuitive event. It was not an explosion, it did not occur inside anything, rather, everything that we are familiar with: left, right, up, down, cause and effect, the stage for all physical laws, was getting larger.
We possess a natural curiosity of the world around us. We want to know how and why things are the way they are.
This curiousity has served us well because it carries significant survival benefits. If we see an event and ask ourselves why it happened or what caused it, we are more likely to spot a threat before it becomes dangerous in the future.
Our curiosity gives us the ability to look beyond the present moment. From it, we have evolved an urge to look for causes, it is an inseparable part of our biology. Because of this, we really can't help ourselves when we attempt to find a cause for creation, it is second nature for us to ask, 'What Caused the Big Bang?'
Any answer to this problem must begin with a key realization: both time and space are contained within the universe and came into existence only AFTER the Big Bang occurred. The cause of the universe must not include them, they are not available to us. It must come from outside our experience.
In this realm, the solution, whatever it is, will seem very strange to us, and it will almost certainly make no sense to our brains because here, it is possible to have an event with no cause. There is no time, there is no before in which the Big Bang could have occurred, there simply is no cause and effect.
We must somehow come up with a solution that exists outside time and space.
GOD MADE IT HAPPEN
For many "God caused the big bang" is a perfectly reasonable response. This seems to help many cope with the unsatisfying prospect of an event without a cause.
The problem of course is that one is then immediately forced to ask, "From where did the creator come?"
If the answer is "he always existed" then we have a situation, from a causality standpoint, that is no more satisfying than a universe that springs forth from nothing. A creator that has always existed is an entity that somehow exists without a cause.
So this answer doesn't solve the causality issue whatsoever.
THIS UNIVERSE IS ONE OF MANY
Further, the idea of cyclical universes doesn't solve it either.
For reasons having to do with entropy and the second law of thermodynamics the process of an ever cycling universe - one that continually expands and contracts - cannot be perfectly efficient. This means that each successive expansion will take a little longer than the previous one.
If each previous universe is, say, half as long as the one that succeeds it, and the one before that half as long, this infinite sum does eventually converge to a universe with zero length with no obvious past and we are back again to at least one big bang starting for no obvious reason.
OUR BIOLOGY BETRAYS US
The real problem with this question of what caused the big bang is ultimately a biological one; our brains have evolved to assume that everything has a cause, we can't imagine any event ever not having one.
But 100 years ago, we couldn't imagine that our galaxy was only one in an ocean of one hundred billion. 200 years ago, we couldn't imagine that the stars were more than 13,000 light years away. 500 years ago, we couldn't imagine that the Earth revolved around the Sun. If our past enquiries into the universe are any guide, the truth of the cosmos is always more than we have imagined.
The answer to the cause of the universe will almost certainly be something strange and, by definition, wholly beyond our experience. Our occluded brains must always be open the answer, especially when asking questions that push the limits of our capacity to understand.