The most widely accepted scientific explanation of the origin of the universe is the:

According to the Big Bang Theory, the universe began about 13.8 billion years ago from an extremely hot, dense and tiny state, and has been expanding and cooling ever since.

Main ideas of the theory:

All the matter, energy, space and time of the universe were once concentrated in an extremely small, hot and dense state.

This began to expand rapidly. In the first tiny fraction of a second, there may have been an extremely fast expansion called "cosmic inflation".

As the universe expanded and cooled, basic particles formed, then simple atoms of hydrogen and helium.

About 3,80,000 years after the beginning, the universe became transparent, releasing light that we still detect today.

Over hundreds of millions of years, gravity pulled matter together to form stars and galaxies.

The Big Bang was not an explosion in space; it was an expansion of space itself.

Evidence for the Big Bang:

Expansion of the universe: In 1929, Edwin Hubble showed that distant galaxies are moving away from us, and the farther they are, the faster they recede. This is known as Hubble's law.

Cosmic microwave background radiation: Discovered in 1965 by Arno Penzias and Robert Wilson, this faint radiation fills the universe and is the afterglow of the early hot stage.

Abundance of light elements: The observed amounts of hydrogen, helium and lithium match the theory's predictions.

Open questions:

• The nature of dark matter and dark energy, which together make up about 95 percent of the universe.
• What happened at the very beginning, if anything, "before" the Big Bang.

Why not the other options?

• ✗The Steady State Theory said the universe has no beginning; it was largely abandoned after 1965.
• ✗The Tidal Theory and the Planetesimal Theory tried to explain the origin of the solar system, not the universe.