Our current science is unable to investigate times earlier than 5.4x10-44 seconds, the Planck time, after the start of the Universe. This time is a decimal point followed by 43 zeros then 54; A very short amount of time. It is the time taken for light to travel the Planck Length, which is approximately 1.6 x 10-33 cm. At the end of the Planck time, the temperature of the Universe was about 1032K. The Science - Introduction contains a complete description of this scientific numbering system, here is a reminder.
Assuming that scientists verify the theory of supersymmetry, then the four forces of nature that we see today as separate forces, Gravity, the Strong Nuclear Force, The Weak Nuclear Force, and Electromagnetism, are unified into a single force during the Planck time. At the end of this period, the gravitation force separated from the other three forces, leaving them combined into a single grand unified force; for a while.
The equations of General Relativity suggest that the Universe started as a singularity; that is, infinite density in an infinitely small space. Scientifically, this is not acceptable, and most scientists agree that General Relativity breaks down below the Planck Length due to random quantum mechanical fluctuations. Quantum physics places limits on size and density, and suggests that the expansion started with the entire Universe in a space with the diameter of the Planck length, and with the density at around 1094 grams per cubic centimeter. Quantum physics allows this super-dense Universe to appear out of nothing at all as a vacuum fluctuation. As its gravitational energy is negative, and its matter energy is positive, they cancel each other so the Universe would have zero net energy. In Quantum mechanical terms, this means that having come into existence, it could last forever as there is no "energy debt" to be repaid! As we shall see, it would have been Cosmic Inflation that prevented this cosmic "egg" from almost immediately collapsing into a black hole.
During the Planck time, the Universe was dominated by Quantum gravity; and as we have no cohesive theory of Quantum gravity, we cannot really understand what happened, so the scenario given above is conjectural. However, it does suggest a methodology that would give rise, over billions of years, to what we see around us today in the Universe.
The Planck Epoch - Up To 5.4x10 -44 Seconds
Astronomy & Cosmology -
The Early Universe
(or "What Banged in the Big Bang?")