The Big Bang theory explains the early development of the universe very well.
The early universe would have been composed only of spacetime and matter that wouldn't have begun to condense for millions of years, forming the first objects in the universe.
Galaxies are gravitationally bound by what scientists classify as a super massive black hole in the center of them.
They could have formed from black holes drawing matter to them or out of dense galactic clouds of gas that developed a black hole.
A star is born from a relatively dense cloud of gas contracting under gravity, becoming incredibly dense till coalescing in a sphere.
The extreme amount of heat and pressure this generates causes nuclear fussion which is why a star burns.
The process of star formation leaves access gas that revolves around the star, and if not absorbed will give rise to solar systems.
Planets form out of the accretion of matter that revolve around a star after its formation.
The heavy elements throughout the universe and that help make up our solar system were made by earlier generations of stars through their nuclear reactions.
The earth formed about 4.5 billion years ago, how it got to where it is now is a subject of geology. In the earlier conditions of earth, simple organisms were able to develope and evolved and adapted into more complex creatures over millions of years.
Humans evolved from hominids, that evolved from earlier primates, that evolved from earlier mammals, that evolved from mammal-like reptiles, that evolved from amphibians, that evolved from protoamphibians(acanthostega), that evolved from ancestral fish...
...and all the way back to the single-celled organisms.
There are lots of good articles about this online, but in a nutshell...
1. As the universe expanded early on, it cooled down
2. This allowed the formation of hydrogen
3. Unevenness in distribution of hydrogen resulted in uneven gravitational pull
4. Hydrogen was attracted to the closest/most powerful center of mass.
5. Black holes formed at these points
6. Hydrogen that had not been sucked into the black holes began orbiting in ever tighter orbits.
7. Stars formed in a manner similar to how the black holes formed
8.. The early stars created heavier elements and then exploded
9. New stars formed (such as ours) from the remnants of earlier stars.
10. Since the galaxies now had heavier elements, planets formed around some of the stars
11. ...and here we are
to answer your question directly, the big bang expanded by matter and antimatter colliding into each other and becoming annihilated. because matter and antimatter have mass, and now they do not exist because they annihilated, then you can plug in einstein's formula of e=mc(2). In other words, energy was released to replace the mass of the matter and antimatter that killed each other. This energy was used to create the inflation (expansion) of the universe also known as the big bang. The energy created the space of the universe So, you see, the big bang was the result of all the annihilation that occurred between matter and antimatter. However, once this war was over, there was a little more matter remaining. this matter became everything in the universe that occupied this space.
It depends on if you believe in the Theory of Catastrophism and Uniformatarianism
I wonder who created the 'thing' that went 'bang' !