If you wanted to describe what the universe is made of in one word, the best answer is hydrogen. About 75% of all regular matter in the universe is hydrogen atoms. Most of the rest is helium. Together, those two elements make up almost everything. All the other elements - the carbon in your body, the iron in your blood, the gold in jewelry, the silicon in computer chips - add up to a tiny minority.
Hydrogen is the simplest possible atom. It has just one proton in its nucleus and one electron orbiting around it. Nothing simpler can exist as a normal atom. In a sense, hydrogen is the foundation that all other matter is built from.
Stars run on hydrogen. Deep in their cores, intense heat and pressure smash hydrogen atoms together, fusing them into helium and releasing enormous energy in the process. That’s where sunlight comes from. Every other element heavier than helium was eventually built up by stars fusing lighter elements together over billions of years. So next time you see a sunset, every photon of light started as hydrogen being squeezed into helium 8 minutes earlier - and most of the rest of the universe is the same simple stuff just sitting around.
