**Big Bang Theory**: The most widely accepted explanation for the origin of the universe is the Big Bang Theory. It proposes that the universe began as an extremely hot and dense point around 13.8 billion years ago and has been expanding ever since.
**Cosmic Microwave Background**: This is the afterglow radiation from the Big Bang, detectable in all directions of the universe. It provides evidence for the Big Bang Theory and offers insights into the early universe's conditions.
**Formation of Matter**: In the first few minutes after the Big Bang, the universe cooled enough for protons and neutrons to form. These particles eventually combined to form simple nuclei, leading to the formation of hydrogen and helium.
**Nucleosynthesis**: This process occurred during the first few minutes after the Big Bang, resulting in the creation of the lightest elements (mainly hydrogen and helium) through nuclear reactions.
