Simulation theory a.k.a matrix theory is a modern hypothesis that describes that we are in a computer simulation of pretty advanced digital architecture and being governed by some higher civilization or intelligence. This may sound like modern terminology, but it has got ancient roots.
Before heading to Simulation cum matrix theory, let us first get through brief knowledge about the formation of our universe.
In 1920, Edwin Hubble studied outside of milky way galaxy through a huge telescope and found numerous other galaxies like Andromeda galaxy. Meanwhile, he observed that our universe is expanding rather than being motionless and thereby the concept of big bang theory was coined. If the universe was expanding, then there would be a time when everything was still and in a single incomprehensible tiny ball of infinitely small volume, infinite density, and temperature.
The explosion took place in a tiny ball resulting huge expansion of it. And the aftermath of that expansion formed our matter, antimatter, stars, planets, space, time, and everything else. Recapitulating all these, then our entire universe is the result of the big bang. Before the big bang, there was no concept of space and time. What existed before time. When there was no time, how could one use the word before or after. Big bang theory may sound absurd but is the most acceptable theory in detailing the origin of universe in the Scientific World.