The Big Bang Theory is a cosmological model explaining the origin and evolution of the universe. According to this theory, the universe began approximately 13.8 billion years ago from an infinitely dense and hot state, expanding rapidly to create everything we see today.

Key aspects of the Big Bang Theory:

• The universe began from a singularity - an infinitely small point with infinite density and temperature
• The rapid expansion, known as cosmic inflation, occurred within the first fraction of a second
• The universe cooled enough to allow the formation of subatomic particles, and later atoms
• Gravity caused matter to clump together, forming stars and galaxies
• The universe continues to expand at an accelerating rate due to dark energy

Supporting Evidence:

• Cosmic Microwave Background Radiation - the afterglow of the Big Bang
• The abundance of light elements (hydrogen, helium, lithium) in space
• Hubble's Law and the observed expansion of the universe
• The distribution of galaxies and large-scale structures

Timeline of the Universe:

• 0 seconds: The Big Bang begins
• 10⁻⁴³ seconds: Planck Epoch - quantum effects of gravity dominate
• 10⁻³⁵ seconds: Cosmic Inflation begins
• 3 minutes: First atomic nuclei form
• 380,000 years: First atoms form, universe becomes transparent
• 400 million years: First stars ignite
• 13.8 billion years: Present day

Current Mysteries and Ongoing Research:

• The nature of dark matter and dark energy
• What existed before the Big Bang
• The ultimate fate of the universe
• The origin of cosmic inflation
• The matter-antimatter asymmetry problem