We don't know what caused it but we are working on it
Because there is no earlier, the big bang marked the beginning of time too (in this universe)
Since we don't know what caused we don't know the mechanism either although an explanation could be the collision between the membrane of our universe with another going by string theory
We are guessing it did happen somewhere else in the multiverse
You can say nothing since time (in this universe) began with the big bang
To be honest.
We will never ever know what EXACTLY happened.
unless something like this happens again (which would cause our death probably)
or we invent a time machine.
GREAT
more to think about
You are correct the truth is we might never know what really caused it we can just hypnotize about it. I hope someday someone would be able to demonstrate us the contrary.
"hypothesize" - though maybe all are hypnotized... :)
We will know what exactly happened as soon as we will have Grand Unified Theory. Time machine isn't required. Moreover that machine can't help us, because no particles can survive in enviroment with temperature above 10^14K.
"why did it happen here ?
Why didn't it happen somewhere else ?"
Because this happen everywhere in our Universe. Big bang could not happen somewhere else, because there wasn't/isn't somewhere else (so far).