The beginning of the universe is a complex and confusing concept, with many views and hypotheses offered to decipher the multifaceted history of our universe. Yet the two overwhelming ideas that have been presented to explain how the universe began, or the beginning of time as we know it, are science and religion. Notice how I did not use science vs. religion (I will explain this view later).
Victor Weisskopf in "The Origin of the Universe," uses a few key scientific facts that are known about our present universe in order to put to formulate a hypotheses of how the universe began. The first is that the majority of stars in the universe are comprised of 93% hydrogen, 6% helium, and 1% other elements. The second fact is that matter in space is distributed unevenly. The third fact is that the universe is expanding at a slowing rate of speed; thus the universe is slowly cooling. The fourth is that we can only see so far into the universe. The fifth point has to do with the expansion and temperature of the universe. Since the universe is cooling at a rate proportional to the expansion of the universe, if the universe were to contract the temperature would rise. From here Weisskopf asks the question, "What is the big bang"? He proceeds to answer this question with a new discovery in particle physics. The false vacuum, in contras to the true vacuum, is empty of matter, not energy. Therefore when a true vacuum (the empty void before the big bang) is fluctuated into a false vacuum or energy is entered into the true vacuum a sudden expansion of energy is created, in other words the big bang.
After the big bang a true vacuum was formed (our universe), which contained all the light, particles and antiparticles from the false vacuum. As the universe cooled so did the protons and neutrons in it, which later would develop into helium and hydrogen nuclei.
From this point in time the big bang, the uneven distribution of matter from the big bang developed in pockets, which allowed the atoms to condense into protostars.