Humanity has been increasing its collective knowledge of the workings of the physical world for millennia. First clawing and scraping to eke out each discovery, but we progressed — as Isaac Newton admitted — by standing on the shoulders of the giants that came before.
For most of that time, the facts gleaned came from careful observation, coming up with things that were clear after some person noticed what had been there all along.
Technology later helped peer into realms that unaided humans couldn't see, such as the machines that helped Faraday and Maxwell unravel electricity and magnetism.
Obvious or not, eventually it was decided that the amount of matter in the universe was conserved. Of course we didn't know what that quantity was, though it was obviously greater than zero (much) but finite (as a conserved quantity must be).
Energy was harder to pin down, remaining so for most of history. It was eventually decided, however, that the amount of energy was also conserved, even though it could be seen to change its form. (For example, the energy in a pendulum changing from kinetic to potential and back as it swings.)
A paradigm-busting genius (standing on those shoulders, but also contributing his own formidable scaffold for those to come) was Einstein. With just his imagination, he found that energy was so fundamental that even matter is a chunk of it.
When the Big Bang was being discussed, they wondered how all that we see could "come from nothing". Physicists, feeling cornered, decided that gravitational energy was negative, and — surprise! — was equal in magnitude to all other kinds. Thus, there was zero at The Beginning, and zero there remains.
Even so, what caused the Bang? The greatest universal force: Love.
It is not conserved; has it always existed?
300