Which unit is employed by professional astronomers for measuring the vastness between galaxies?

Parsec

For professional astronomers dealing with the most immense scales in the universe, such as the vast distances separating galaxies or large sections within the Milky Way galaxy itself, the parsec is the unit frequently employed. While the light-year is useful for nearby stars, the scale demands an even larger measure. The parsec is defined based on parallax measurements—a geometrical technique used to gauge stellar distances. Quantitatively, one parsec is equivalent to approximately $3.26$ light-years, which translates to roughly $30.9$ trillion kilometers. This unit choice reflects the necessity born from the colossal spatial separation encountered at the galactic and intergalactic levels.