The most widely accepted scientific hypothesis today is that the Universe was a spontaneous quantum event like many that happen to this day and age. This event has no cause, it simply happens because it is allowed to do so naturally by the current laws of physics. This fluctuation has disassociated into the forms of space-time and mass / energy, so that the total energy of the Universe is zero (energy in the form of mass is canceled by gravitational energy). This hypothesis was practically "confirmed" by the observation that the Universe is flat. Only a flat Universe allows it to have zero energy and may have "sprung" from a quantum fluctuation.
Although the scientific hypothesis above is by far the most accepted today to explain the "before the Big Bang", there are some others that still merit some consideration: the hypothesis of the multiverses, hypothesis of the cyclic Universe, the hypothesis of the membranes.