The element that makes up about one-third of the mass of the universe is hydrogen.

Here's why:

* Abundance: Hydrogen is the most abundant element in the universe, making up roughly 75% of its normal matter by mass.

* Stars: Stars are primarily composed of hydrogen, which undergoes nuclear fusion to produce energy.

* Dark Matter: While we can't directly observe dark matter, it makes up a significant portion of the universe's mass. However, it's not composed of any known element.

So, while hydrogen doesn't account for the entire universe's mass, it's still the most abundant element and makes up a large portion of the visible matter we observe.