Final answer:
The true statement about galaxies is that they come in different shapes and sizes. Galaxies' diversity includes spiral galaxies with active star formation, giant elliptical galaxies filled with old stars, and irregular galaxies with less defined structures.
Step-by-step explanation:
The universe is indeed filled with billions of galaxies, each with its own unique characteristics. The correct statement about galaxies is that they come in different shapes and sizes. This diversity can be seen in the spiral, elliptical, and irregular types of galaxies, each with different structures and star populations.
Spiral galaxies like the Milky Way contain both old and young stars and significant amounts of interstellar matter. They usually have spiral arms where new stars are being formed. Elliptical galaxies consist mostly of old stars and contain little interstellar matter. They range from being some of the largest galaxies in the universe to dwarf sizes. Finally, irregular galaxies have far more chaotic shapes and structures and also undergo high rates of star formation.
In conclusion, the vast number of galaxies are not uniform in structure but exhibit a variety of forms, ranging from well-defined spirals and round ellipticals to less organized irregular shapes. This rich diversity of galaxies is a fundamental characteristic of our universe.