There is a thin veil of life permeating the galaxy.  Most of it is lumped together under the category of "exo-plankton", a term that describes a wide array of simple life-forms found living in deep space.  Human explorers first confirmed the existence of exo-plankton in the mid-2000's, when samples from the asteroid belt were found to contain simple viruses.

General types

• Organic molecules and prions
• viruses
• spores, and spore-like cells
• exo-bacteria
• other plant- and animal-like microorganisms

The more complex microorganisms are typically found near the frost-lines of stars where the temperature is in the range where volatile compounds can exist in liquid form. Typically, in the region closer to a star than the inner boundary of the habitable zone, the more complex forms are absent, with only viruses, spores and molecular types present.