Question: Is it really possible to directly detect "dark matter" with a radio telescope? If so, how?

Adam Tuff answered on 14 Mar 2011:

This is the thing about dark matter – it’s “dark” in that we can see it, with any types of radiation. So this is a really good question…if you can’t see a thing, how do you detect it?

Well, this is where Gravity plays a part. Hydrogen is the most common element in our universe, and even neutral hydrogen (hydrogen that isn’t in an excited state) emits radiation – this comes in the form of radiation that has a wavelength at radio frequency (it’s wavelength is 21cm). We can look into the sky and see hydrogen anywhere in our galaxy because it isn’t blocked like visable light is by dust.

We look at how the radiation is doppler shifted (I guess your user name is appropriate!), i.e. how it’s wavelength has been changed depending on it’s speed, and we can build a picture of where all of the hydrogen clouds in our galaxy are, and how fast they are moving.

Now, if Newton’s laws are obeyed, the further a smaller object (a hydrogen cloud) is from a large object (the centre of our galaxy), the slower it moves. When we build our picture of the hydrogen in our galaxy, and look at how fast everything is moving…this isn’t what we see, and in fact all the hydrogen moves at the same speed! So either this means that Newton’s laws “break” at a galactic level, or there is an almost uniform distribution of matter throughout our galaxy.

You can then calculate how much mass you “should” have to see the speeds you observe, and voila! You can estimate how much dark matter is present in our galaxy.

We still don’t know what Dark matter actually is…but that’s another question!