Yes, the universe is mostly empty space. The vast majority of the universe consists of empty regions between galaxies and stars, with relatively low matter density. This is because matter is not uniformly distributed throughout space, but instead clusters into galaxies, galaxy groups, and larger cosmic structures.
Estimating the precise numbers of stars and galaxies in the universe is challenging due to its immense size and the limitations of current observational capabilities. However, scientists have made various estimations based on observations and theoretical models.
Stars: The estimated number of stars in the observable universe is on the order of 10^22 (or 10 sextillion) to 10^24 (or 1 septillion) stars. This estimation takes into account the known galaxies and their average star population, along with extrapolations for unobservable regions.
Galaxies: The estimated number of galaxies in the observable universe is in the range of 100 billion to 2 trillion galaxies. This estimation is based on surveys of the observable universe, which have provided data on the distribution and density of galaxies.
It's important to note that the observable universe represents only a portion of the entire universe, as the universe is thought to be much larger than what we can observe due to the expansion of space. The true number of stars and galaxies in the entire universe could be significantly higher.
While the universe is mostly empty space, the presence of stars and galaxies in that space is what gives rise to the structures and phenomena we observe, such as galaxies, galactic clusters, and cosmic filaments.