Here's why:

* Big Bang Nucleosynthesis: The universe began as a very hot, dense state. During the first few minutes after the Big Bang, the universe cooled enough for protons and neutrons to fuse, forming primarily hydrogen (one proton) and helium (two protons and two neutrons).

* Abundance: Hydrogen makes up about 75% of the normal matter in the universe by mass, with helium accounting for about 24%. All other elements are much less abundant.

While hydrogen is the most common atom, it's worth noting that dark matter makes up a much larger percentage of the universe's mass. However, dark matter is not made of atoms and its composition is still a mystery.