A Macrocosm is the natural universe as a whole, including the biological realms of flora and fauna, weather and celestial objects.
A macrocosm is one of the two types of synecdoche.
Side note: A synecdoche is a figure of speech where a whole represent a part and vice versa. A microcosm is a synecdoche where a smaller part represents a larger whole.
This is one example of a microcosm:
The Department of Defense released an extensive report this evening.
Here, "The Department of Defense" is a macrocosm. The entire Department of Defense cannot release a report. An individual or a small group of individuals usually achieve this. The Department of Defense is a macrocosm because it is a whole that speaks for a part.