Can someone please explain the Big Bang theory?
It’s not exactly something you can explain shortly. Where to begin? There are several different ideas on what may have “caused” the Big Bang and what “happened before” it. The theory posits that some 13.7 billion years ago, there was an incredibly rapid expansion of space-time (not an “explosion” as the word Bang leads you to believe). Think of a balloon being inflated really quickly. The surface of this balloon is what we know as space-time, which includes the four dimensions that we are aware of every day (length, height, depth and time). This expansion was so fast that the universe grew in size several million times in just a few seconds, getting incredibly hot in the process. As the universe continued to expand, it cooled. Super heated particles began to coalesce into first gas, then stars, planets, asteroids and pretty much everything else.
The big bang refers to the event that started our universe 13.7 billion years ago. At that point, for reasons we don’t yet fully understand, all the matter and energy in the universe was compressed into a singularity (small, hot, dense spot) which began to expand. As it expanded, it cooled, eventually cooling enough for solid matter (hydrogen gas) to form, and later lead to the formation of stars, then galaxies, and planets and finally us. It is well-supported observationally and experimentally (look up Hubble, the Doppler effect, and the cosmic microwave background) and it was NOT an explosion.
