Finally, an easy question!

"We don't know"

"The" Big Bang is not an event.

"Big Bang" is the awful nickname of a theory; the nickname was given by an adversary of the theory (Fred Hoyle) in 1949. It has the unfortunate effect of making people think that "the Big Bang" is an explosion event.

Background:

1927: a priest (Lemaitre) who happens to be a great mathematician and a good astronomer, proposes the Primeval Atom Hypothesis: all matter existed in a mass of infinite density that exploded: all matter started flying apart.

Early 1930s: Father Lemaitre proposes a mathematical model showing that space, if it contains mass, cannot remain static: it must either expand or contract. Observation work he did with Edwin Hubble (the American astronomer) showed that the choice was "expand" as opposed to contract. Einstein had a hard time with that (he had predicted a static universe where space neither expands nor contracts). In this model, there is no longer any "explosion" because, by then, scientists understand that matter cannot exist at "infinite density".

After a private meeting betwee the two (Father Lemaitre was humble enough to not gloat), Einstein accepted the model and described his result (the coefficient "lambda" that made space static) as "the biggest mistake" of his life. Einstein and Lemaitre toured the USA to explain the mathematical model to cosmologists. Of course, Einstein used the opportunity to explain his Relativity theory at the same time.

1948: scientists Alpher and Gamow use the priest's model to build a theory to explain the effect of the expansion of space on the energy content of the universe:

Same total amount of energy + more space to spread it out = the energy density (temperature) goes down with time.

And they explain how the initial energy became the first elements (nucleosynthesis).

1949: Fred Hoyle comes up with a different theory (Steady State) whereby the expansion of space does NOT change the density of the energy -- new energy is being constantly created. In his theory, the universe is eternally the same. Eternal universe = no creation = no creator (Fred was a proud atheist - that is why he hated the priest's idea). In 1949, during a radio interview, Fred came up with the nickname "Big Bang", for the theory that came from the priest's idea.

1964: with new and imporved ratio-telescopes, evidence starts to come in and shows that Steady State is invalid. Furthermore, the evidence confirms the predictions of Big Bang. Unfortunately, by then, the nickname has stuck. Even though we know the theory contains no explosion, everyone still calls it "Big Bang". Father Lemaitre was still alive in 1964. By then, the Church had rewarded him for his discovery by promoting him to the rank of Domestic Prelate (equivalent to a bishop, but without the need to look after a See). The Church was very happy with the theory that came out of Monsignor Lemaitre's model.

The Planck Time - the initial moment of the expansion as explained by the Big Bang theory - is what most peope (usually non-cosmologists) refer to when they talk of THE Big Bang, thinking it was an explosion.

It is simply the first moment in the domain of the mathematical model. The main variable is time, and if you push the model back to that time, you reach a moment when the density was so high that we no longer understand how things work.

At the Planck Time (13.8 billion years ago):

-- the initial energy already existed

-- space was already expanding

-- energy density was NOT infinite (the number is known)

-- matter did not yet exist (it comes later)

The problem really is that we do not understand how time "flows" at higher density. Some think that time and gravity become "the same thing" at higher density. It is a unification threshold.

In other words, the word "before" does not (yet) make sense at that special moment.

Your question "What caused the Big Bang?" is about what happens "before", and we don't know.

There are roughly a half-dozen ideas about what COULD have happened before. One of them (the one promoted by Father Lemaitre) is that [his] God created the universe during the tiniest fraction of a second "before" the Planck Time. The other extreme is that nothing happened: space has been expanding forever and it is only 13.8 billion years ago that the density became "low enough" for us to perceive the flow of time.

In between, there are things like M-theory, String theory, the idea that n-branes (static subspaces of "n" dimensions existing in a higher-dimension Universe) collided to cause the expansion of three of the "n" dimensions.

Some of these ideas require the universe to be finite. Because their mathematical models are easier, they are more popular, causing some people to hope that the universe is NOT infinite in spatial extent. But other models still allow our whole universe to be infinite. Do not confuse it with the Observable Universe (the portion we can perceive) which is definitely finite.