1. Hydrogen (H): With an atomic weight of approximately 1 atomic mass unit (amu), hydrogen is the lightest element and the most abundant element in the universe.

2. Helium (He): With an atomic weight of approximately 4 amu, helium is the second lightest element and the second most abundant element in the universe.

While there are other gases in the universe, such as oxygen, nitrogen, and carbon dioxide, they are significantly heavier than hydrogen and helium.