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Theory:

Scientists believe that the universe began with a tremendous explosion known as the Big Bang. According to the Big Bang theory, all of the matter in the universe was concentrated into a single point of hot dense matter.
 
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Big Bang

Around \(13.7\ billion\ years\) ago, suddenly, an explosion occurred, and all the matter were ejected and expanded in all directions. In other words, the universe expanded from a size smaller than an atom to a size larger than a galaxy in a fraction of time.
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Origin of elements:
The simplest elements like hydrogen and helium were formed in the Big Bang and makeup nearly all the matter in the universe that we understand. The rest of the elements, such as oxygen, carbon, calcium, iron, and silicon, are created in the core of stars.
 
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The core of a star
 
The gravitational force holds the stars together and keeps the elements buried deep into their cores. These fundamental elements of planetary systems are released throughout the universe when these stars burst.
Reference:
https://upload.wikimedia.org/wikipedia/commons/f/f0/Massive_star_cutaway_pre-collapse_%28pinned%2C_8M%2B%29.png