Era 2: From Quarks to Nucleons to Nuclei
As the Universe expands, it also cools. With this cooling, the intense concentration of energy from the beginning of the Universe spreads out and cools. Although energy can take the form of matter, and matter can convert back into another form of energy, over a cosmically short period of time, the Universe had cooled enough that a significant amount of energy stayed in the form of matter.
E = mc2
Matter is just one of the many forms of energy. The equivalency of these two seemingly unrelated things is reflected quantitatively in Einstein's famous equation E = mc2. In other words, it takes a lot of energy to create a little bit of matter. Conversely, it only takes a little matter to get a lot of energy. A one gram paperclip could be converted to enough energy to run a 100 W light bulb for 28479 years!
E stands for "energy," m for "matter," and c for the speed of light, which is
approximately 3 x 108 m/sec.