We can detect 21-cm emission from other galaxies as well as from our own Galaxy. However, 21-cm emission from our own Galaxy fills most of the sky, so we usually see both at once. How can we distinguish the extragalactic 21-cm emission from that arising in our own Galaxy? (Hint: Other galaxies are generally moving relative to the Milky Way.)

Explanation:

Galaxies are in constant motion with respect to each other . For example Andromeda galaxy is approaching our galaxy ( milky way ) at about 110 km /s . So we will observe blue shift in the spectrum of radiation coming from  this galaxy . In this way, we can distinguish between radiation coming from our galaxy and that coming from other galaxy . Spectrum of radiation coming from other galaxy must  have either red or blue  shift .

a electromagnetic waves