The Big Bang is a theory that explains how the universe expanded from a single point. The Big Bang occurred 13.7 billion years ago. All the matter, energy, and light; were all compacted into an infinitely dense point. The universe then expanded tremendously.
As the seconds passes; this condensed energy was spread out, and the forces such as gravity can into existence. Around 300,000 years after the Big Band, the universe was abundant with floating particles. These particles then slowly came to each other. The most simplest elements came into existence, most found was hydrogen (with 1 proton) and helium (with 2 protons).
The different atoms were able to form and then these particles became more and more condensed. Soon matter was formed. The early stars were made up of very simple elements and so they had very short lifespans of only millions of years. But the nuclear fusion in the cores' of these early stars slowly created heavier elements; which when the star died, would become a part of a new star.
These stars were created in small groups and attracted other stars. These stars were grouped in irregular shapes. Then the different shapes merged to form the first galaxies.
Then as more galaxies formed, they became grouped in galaxy clusters, and then these clusters were contained in super clusters.
Today, scientist know about a force called dark energy. Dark energy acts like an anti-gravity and does the opposite of gravity. Dark energy is currently the cause of the expansion of the universe. The universe is expanding at an accelerating rate today.
With the discovery of dark energy, many scientist now believe the the universe will continue to expand forever and all of the matter in the universe will eventually decay within a trillion years.