They were the first ones to show up. You see, right after the Big Bang, the universe was very hot and dense. So hot and dense that only the simplest atoms could exist: hydrogen and helium. Hydrogen has one proton and one electron, while helium has two protons, two neutrons, and two electrons. Anything more complicated would be torn apart by the intense energy.
For about three minutes, the universe was a giant nuclear reactor, fusing hydrogen into helium and releasing energy. This process is called nucleosynthesis, and it's how stars make their light and heat. But after three minutes, the universe cooled down enough that nucleosynthesis stopped. The result was a universe filled with about 75% hydrogen and 25% helium by mass . The rest of the elements were either too rare or too unstable to matter.
Other elements like carbon, oxygen, iron, gold, etc.., were made later, by stars. Stars are basically giant balls of hydrogen and helium that fuse them into heavier elements in their cores. The more massive the star, the more elements it can make. For example, our Sun can only make carbon and oxygen, while massive stars can make iron and beyond.
But even stars have their limits. When they run out of fuel, they die in spectacular explosions called supernovae. These explosions not only spread the elements they made into space, but also create new ones by smashing atoms together at high speeds. This process is called supernova nucleosynthesis , and it's how we get elements like gold, silver, uranium, etc.
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