Answer:
Gravity/Cosmic Inflation theory (It's really controversial)
Step-by-step explanation:
According to my prior knowledge and previous research, the Big Bang Theory models that (apparently) the universe, 13 billion years ago, was simply a small speck jam-packed into one very small space (a singularity).
Eventually, gravity is what caused this speck to explode at a great speed (light speed to be exact) and as this happened -- matter and galaxies and clusters were 'somehow' created.
According to this same theory, our universe is still expanding. Not sure if this answer sated your knowledge, but tell me if you need more information!