While there are two main domains of prokaryotes: bacteria and archaea, bacteria are far more abundant and diverse than archaea. They are found in almost every environment on Earth, from the deepest oceans to the highest mountains.

Here's why bacteria are considered the most common:

* Abundance: They are estimated to be the most numerous organisms on Earth, outnumbering all other organisms combined.

* Diversity: They encompass a vast array of species with diverse metabolisms and adaptations.

* Ubiquity: They can be found in almost every environment, from soil and water to the human gut and even extreme environments like hot springs and hydrothermal vents.

While archaea are also fascinating and play important roles in ecosystems, their numbers and diversity are significantly lower than those of bacteria.