The composition of the universe evidence indicates the universe is composed mostly of which elements?

• A. Neon and argon
• B. Heavy metals
• C. Hydrogen and Helium
• D. Oxygen and carbon dioxide

Answer: (C)

The universe is dominated by light elements produced in the Big Bang. Observations show that most of the baryonic mass lies in hydrogen and helium — roughly three quarters hydrogen and about a quarter helium, with only a small fraction of heavier elements remaining. This is why hydrogen and helium are the correct answer: they are by far the most abundant elements on a cosmic scale.

Neon and argon are noble gases found in stars and planetary atmospheres, but their overall abundance is tiny compared with hydrogen and helium. “Heavy metals” (all elements heavier than helium) exist in stars and interstellar gas, but they make up only a small portion of the universe’s mass. Oxygen and carbon dioxide do appear in space, but they are not the majority; oxygen is more abundant than many other elements, yet still far less than hydrogen and helium, and carbon dioxide is just one form of carbon-bearing molecules among many.