What we do is we observe radio waves coming from the universe. The galaxy is giving off huge amounts of radio waves. That’s how it was discovered, because it was messing up people’s radio communication. It turned out it was the galaxy, and as a result we don’t use these frequencies any more. We use either really low frequencies protected by the ionosphere or we go to higher frequencies where the galaxy is not so bright. We know what the galaxy, or the radio sky, should look like. We have a map. So, we look at where radio waves are missing. We measure what we actually see and the difference is the absorption.
– Derek McKay, KAIRA (see Radar) Observer-in-Charge, in discussion with HH and AM, 2018, Tromsø