Question

How would you measure the speed at which a galaxy is traveling?

Answer 1

Galaxies are receding from one another at the rate 72 km/sec/mega parsec. This is astronomer Edwin Powell Hubble's empirical formula..

Explanation:

This Hubble's constant is also a measure for the expansion of our universe.
mega parsec = million parsec and 1 parsec = 3.26 light year,
i light year = 206264.8 AU = 3.085677 E+13 km, nearly