everything that exists, especially all physical matter, including all the stars, planets, galaxies, etc. in space

Is there intelligent life elsewhere in the universe? 宇宙中其他地方存在有智慧的生命吗？

a universe that could be imagined to exist outside our own

Scientists have speculated about the possibility of parallel universes. 科学家在思考平行世界存在的可能性。

the world, or the world that you are familiar with

The characters in his novels inhabit a bleak and hopeless universe. 他小说中的人物生活在一个悲凄无望的世界里。

His family is his whole universe (= everything that is important to him). 他的家庭就是他的整个世界。