That is the basic fundamental question behind our existence.
In essence it is believed that there was never nothing, because if nothing exists then, there is no objective reality ie: we wouldn't exist....
It is believed that due to gravity increasing compression of particles led to a Big Bang and the creation of all the elements and creations of the universe today.
When the universe stops expanding as it is now it will begin to compress again due to gravity, and once all the matter has been compressed to seemingly infinite density, another Big Bang will occur and the universe will be created again.
It is called the Cycling Reality Theory, read up on it if your interested.
Hope this helps...