E = mc2. Einstein’s famous formula tells us that energy can be converted to mass. But there’s a catch – the mass must be equal amounts matter and anti-matter.
Immediately following the Big Bang the energy released was converted into mass, equally balanced between matter and anti-matter.
According to the laws of physics as they are currently known that balance would have been maintained. If that were true, shortly following the Big Bang every matter particle would have found a corresponding anti-matter particle, combined with it, and in a reverse of Einstein’s equation (mc2 = E) converted all the mass back into energy. The Universe would have been filled with light and nothing else.