In the beginning, well, we have no way of describing the beginning, for our physical laws do not apply to a realm of infinite density devoid of space. We have no knowledge of a time before our Universe existed, or a space other than the space created within our Universe. In the beginning, there was no space.
The Universe began to expand, and space was created. The space had no center and no end. It was infinite, and ever growing. And in that space was a dense sea of matter. But the matter was not of the form to which we are accustomed today. Exotic particles that today are seen only in particle accelerators were common at that time. They teemed through the Universe, continuously colliding and interacting and transforming from one type of particle to another.
In an instant that was as small a fraction of a second as a spec of dust the width of a human hair is to the volume of the Earth, the matter and antimatter annihilated. For some unfathomable reason, there was more matter than antimatter in the Universe at that time, so only the matter was left behind. The quarks which make up the atoms in our bodies were formed at that time.
With the creation of quarks, the Universe began an extremely rapid expansion of its space. As suddenly as that rapid expansion began, it ended. The Universe was now a small fraction of a second old - a fraction like the size of a grain of sand to the volume of the Earth.
For several minutes, the Universe expanded and cooled, until it was cool enough for the quarks to bind together into protons and neutrons, which are the building blocks of atoms. The hydrogen and most of the helium in our Universe today was created in this first few minutes. The atomic nuclei at that time swam in a sea of rapidly moving electrons. Light in that early universe was intimately intertwined with the matter, since it could not travel far without colliding with the electron sea. The Universe was opaque.
For 10,000 years, the Universe continued to cool and expand, until it was cool enough that the electrons could combine with the atomic nuclei. At this time, the light was divided from the matter, and the Universe was suddenly transparent. The light released from that division is still traveling through the Universe today, and is called the Cosmic Microwave Background.
Though atoms were made and light was freed from those atoms, there were not yet galaxies or stars or planets, or solids of any kind. These entities would form over the next 100 million years of the Universe, due to the unceasing pull of gravity, ever pulling together the pieces of matter afloat in the Universe.
And so ends the second day of Genesis.