The universe is sometimes described as an isolated system. Why?

Answer and Explanation : The universe means it includes everything, even the things which we can not see is an isolated system because universe has no surroundings. an isolated system does not exchange energy or matter with its surroundings. Sometime universe is treated as isolated system because it obtains lots of energy from the sun but the exchange of matter or energy with outside is almost zero.

the total energy of an isolated system is always constant means total energy of universe is also constant there is no exchange of matter or energy in an isolated system