13.7 billion years ago
The Big Bang occurred, creating the Universe we see today. In its earliest stages the Universe was very hot and very dense. So hot and dense, in fact, that even subatomic particle such as electrons and protons were being created and destroyed all the time. Most of the Universe was composed of photons, particles of light. The matter the stars, planets and we are made of accounted for a tiny fraction of the energy. With all this going on, the Universe was opaque, meaning that light couldn’t get very far before being stopped or redirected by something.
The Universe wasn’t completely uniform – there were tiny “quantum” fluctuations in its density and temperature. They existed simply because nothing can be completely uniform when you get down to the smallest scales. All matter, and even light itself, is made up of individual particles and it is impossible to have half a particle.