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.)

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.