Scientists use several units to measure large distances, depending on the scale they're dealing with. Here are some common ones:

* Kilometers (km): This is the standard unit for large distances on Earth, like the distance between cities.

* Astronomical Units (AU): This unit is used for distances within our solar system. One AU is the average distance between the Earth and the Sun.

* Light-years: This unit measures the distance light travels in one year. It's used for distances to stars and other celestial objects outside our solar system.

* Parsecs: This unit is also used for interstellar distances. One parsec is equal to 3.26 light-years.

* Mega-parsecs (Mpc): This is used for measuring distances to galaxies and galaxy clusters, which are incredibly far away.

The choice of unit depends on the specific context. For example, a scientist studying the distance between galaxies would use megaparsecs, while a scientist studying the distance between planets in our solar system would use astronomical units.