Universe

The universe comprises all of existence: all forms of matter and energy, and the structures they form, from sub-atomic particles to entire galactic filaments. Since the early 20th century, the field of cosmology has established that the universe has been expanding for 13.8 billion years, starting from a dense fireball in an event called the Big Bang. The observable portion of the universe is approximately 93 billion light-years in diameter at present. The total size of the universe is not known.