Georges Lemaître, a Belgian cosmologist and Catholic priest, first introduced the idea of the Big Bang in 1931.  Since then, it has become the leading explanation among scientists about how the Universe began.  According to the theory, the expansion of the observable Universe began with the explosion of a single particle at a definite point in time.  This explosion created all matter and energy in the Universe.  One second after this explosion, NASA estimates that the temperature of the Universe was about ten billion degrees Fahrenheit.  This early Universe contained particles known as protons, neutrons, electrons, photons, neutrinos, and anti-electrons—or positions.  Because the temperature of the Universe was so high, free electrons floated all around, scattering photons—light particles—all around (think of the way that clouds or raindrops cause light to scatter in different directions).  Because of this scattering of light, the Universe was opaque.