What is the Universe made of?

The universe is made up almost entirely of hydrogen and helium. These are the two lightest elements. All the rest of the matter in the universe is very rare. Elements such as silicon, carbon and others are concentrated into clouds, stars and planets. The universe is held together by four invisible forces. Gravity and Electromagnetism are the two familiar forces. The other two kinds are strong and weak Nuclear forces. These operate only inside the incredibly tiny nucleus of atoms, holding the tiny particles together