The universe, which is everything and anything that exists, is still a mystery to scientists. It is made up almost entirely of the two lightest elements, which are hydrogen and helium. 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 being the two most familiar. The other two kinds are strong and weak nuclear forces. These operate only inside the incredibly tiny nuclei of atoms, holding the tiny particles together.
Nobody knows exactly how big the Universe is, what shape it takes, where it comes from or what it is expanding into. If we could go far enough into space maybe we could understand more.