The most commonly used unit to measure the distance between galaxies is the "light-year" (ly). A light-year is the distance that light travels in one year in a vacuum, which is approximately 9.461 trillion kilometers (5.879 trillion miles).
Since the vast distances between galaxies are incredibly immense, it is more practical to use light-years rather than smaller units like kilometers or miles. Galaxies are typically millions or billions of light-years apart, and expressing these distances in smaller units would result in unwieldy numbers.