What caused the Big Bang?
The origin of the components of the universe is unknown to science. Many point to the big bang as the origin of everything, but the big bang does not address the issue of origin. The big bang is the process of expansion and is still visibly in progress. (Many times those who refer to the big bang are speaking of the very early portion.) The evidence found so far does not indicate the origin of the components for the universe, nor does the evidence tell us anything of events occurring before its expansion. Rather than repeatedly reciting this qualifying sentence, the big bang is often simply referred to (incorrectly) as the beginning or "creation" of the universe. Television documentaries do not bother to make the distinction. Though not technically correct, for most practical discussions of the physics of the universe, this is okay. For discussions on the origin of the universe, the error is unacceptable. Guesses and ideas on the origin of the universe are plentiful but unsubstantiated. There are many religions that have their own explanation although most reject all other explanations but their own. Picking one of these at random to believe does not seem like a rational idea. Although some ideas have the big bang as the start of the universe, I don't know of any scientific requirement for or against an infinitely long past. .
What caused the Big Bang?
The big bang is the expansion of the universe and is still visibly in progress. The rate of expansion is increasing against the force of gravity, so we know some force is causing it. We have labeled this force "dark energy". In this case, "dark" is used to express the fact that we know almost nothing about this type of energy other than its quantity. (One should not confuse a possible origin of the universe with the big bang. There is no evidence for or against the universe having an infinitely long past.)
What process caused the Big Bang?
Here is the most complete explanation I've found (and no, "God did it" is not an explanation). According to string theory, there are ten spacial dimensions (plus one for time). A dimension is simply a direction (Left-Right, Up-Down, Forward-Back, etc.). Before the time of the Big Bang, all of them were extremely small with virtually no room for matter to move around. Seven of them are still that way today. At the time of the Big Bang, the natural quantum fluctuations dictated by quantum mechanics caused the three spacial dimensions I mentioned above to expand very slightly. Although gravity always attracts particles together over the enormous physical distances we can see today, when all matter is compressed together in one tiny point it causes gravity to become REPULSIVE. This is shown by the equations of string theory. With all matter in one location pushing against each other, the three dimensions that were slightly stretched were forced to expand outward extremely rapidly. This is what we now refer to as the Big Bang.
Did dark energy cause the big bang?
THE INFLATIONARY UNIVERSE is the most appropriate explanation, it seems , about the cause of the Big Bang, which explains about the period before the Bang of the Big Bang.According to this theory, quantum fluctuations at an infinitesimal point(I.e. less than 10^-106 cum) led to the creation of mass and energy, and from that emerged the whole Universe that we see now.Starting point is quantum fluctuations and ahead. If you read the article, it talks of repulsive gravity as the possible cause of pulling apart of that tremendous mass - energy available at the Bang of Big Bang.This article also mentions about dark-energy but it avoids direarly equating repulsive gravity as to dark energy.But if we ponder over our current Universe and consider the Universe at Big Bang as the minitiature version of the Universe of Now, it seems that there is no problem in equating repulsive gravity to dark energy.But what is causing this dark energy / repulsive gravity. According to my interpretation of reality of Universe explained at BIG QUESTIONS , this force was / is caused by repulsion between Universe and Anti-Verse.However, alternate explanation which are more comfortable to accept are also given in following answers:Tufail Abbas's answer to How do we know that our universe is expanding and not contracting? Near galaxies would come toward us and far galaxies would appear to speed up away.Tufail Abbas's answer to Could it be possible that gravity is a property of space? Could gravity exist due to any acceleration in space, not mass?Although, I am confident about my explanation, readers are advised that if they are not convinced , they should thoroughly examine / test these predictions , before believing.