explain the universe

universe.py

class Universe:
    def __init__(self, age, size, galaxies):
        self.age = age  # Age of the universe in years
        self.size = size  # Size of the universe in cubic meters
        self.galaxies = galaxies  # List of galaxies in the universe

    def expand(self, expansion_rate):
        # Simulate the expansion of the universe
        self.size *= (1 + expansion_rate)

    def create_galaxy(self, name, stars):
        # Create a new galaxy and add it to the universe
        galaxy = Galaxy(name, stars)
        self.galaxies.append(galaxy)

class Galaxy:
    def __init__(self, name, stars):
        self.name = name  # Name of the galaxy
        self.stars = stars  # Number of stars in the galaxy

    def describe(self):
        # Describe the galaxy
        print(f"Galaxy: {self.name}")
        print(f"Number of Stars: {self.stars}")


# Create the universe
our_universe = Universe(age=13800000000, size=3.5e80, galaxies=[])

# Expand the universe
our_universe.expand(expansion_rate=0.0001)

# Create a few galaxies
our_universe.create_galaxy("Milky Way", stars=200000000)