No one knows. But people have grappled with the question since the beginning of the Big Bang Theory. Einstein argued that time was born with the Big Bang, so there was no 'before' the Big Bang.
The most popular theories about the earliest phases of the Big Bang describe a process called inflation that started from a so-called so-called 'false vacuum', which is a state of empty space filled with a high energy density.
Within the tiniest fraction of a second, the visible universe increased in size from a tiny speck, smaller than an atom and filled with immense energy, to billions of kilometers across or more.