Simplify the following text for a Grade 8 native English-speaking person.
Original: The universe is commonly defined as the totality of everything that exists , including all physical matter and energy , the planets , stars , galaxies , and the contents of intergalactic space , although this usage may differ with the context ( see definitions , below ) .
Simplified: The universe is commonly defined as everything that exists . It includes all kinds of physical matter and energy , the planets , stars , galaxies , and all the contents of space .