Why is the universe so cold?

because the sun make a world warm and the sun is not that big enough to warm up all the universe.

I don't agree. The universe is technically a vaccum that can expand or shrink and therefore this vaccum is not entirely filled with gases that can trap heat and stars or objects producing friction which creates heat.

Besides the previous person's awnser is not valid. The sun is a star and the universe is filled with many trillions of stars which are being added to constantly which is why the universe does not have one sun.