The Universe is all of time and space and its contents. It includes planets, moons, minor planets, stars, galaxies, the contents of intergalactic space, and all matter and energy. The observable universe is about 91 billion light-years in diameter. The size of the entire Universe is unknown.
The word universe derives from the Old French word univers, which in turn derives from the Latin word universum.

What How Why

Why does iron go rusty?

Rust forms on the surface of iron objects when they are exposed to the air. It is the product of a chemical reaction called oxidization. The more the object is exposed to the air, the more rapidly rust spreads.