Re: How to detect Dark Matter?

Hi Jeff,

Now, if I knew how to detect dark matter I would be very rich by now. 
I'm not. Dark matter is one of those areas of physics which is mainly 
speculation, albeit well founded. It's also a question of taste. The 
reason why one may want dark matter in the universe is so it doesn't 
go on expanding forever. Some people would like this to happen, 
others prefer a universe which will collapse. At present we have not 
found (or predicted) enough matter to allow the gravitational attraction 
of the universe to cause it to collapse. But if you don't want the 
universe to go on expanding and become big and boring, you'll want 
to find some dark matter. Basically (I imagine you know this) dark 
matter is stuff we cannot observe easily. There are a few examples of 
what could be dark matter which may or may not exist, be massive or  
be abundant enough to cause the universe to collapse:

Neutrinos:
These are fundamental particles which carry the extra energy and 
angular momentum (spin) in some interactions (particularly those 
involving the weak force). They have been detected, but it has not 
been proved that they have a rest mass or not. They are plentiful in 
the universe, our Sun spews them out all the time, some are 
probably passing through you now, but they interact so infrequently 
that one can pass from one end of our galaxy to the other 12 times 
before it interacts. Neutrinos come in 3 flavours, electron neutrinos, 
mu neutrinos and tau neutrinos and these may have different 
masses, if any. 
WIMPs:
this is an acronym for Weakly Interacting Massive Particles. These 
particles only interact with the weak force (which mediates decay of 
nuclei and likes to break rules occasionally) and gravity. This means 
they do not emit or absorb light and so are 'dark'. As a result they are 
very hard to detect, if they do exist. Attempts to detect them are made 
in large underground experiments -- no luck yet.

X and Y bosons:
these are thought to cause proton decay, and may have masses of 
10^15 GeV (the proton has a mass of 939 MeV). Exchange particles 
such as these would only travel about 10^-31 m and so would not 
exist for long, but the mass/energy must come from somewhere.It 
does seem probable that they exist, as current theories predict more 
exchange particles than we are currently aware of.
Scientists are fairly convinced of the existence of dark matter 
as observations of the orbits of stars in galaxies and galaxies
within clusters of galaxies show that there is more matter than can
be accounted for by the light produced.

So the short answer to your question, unfortunately, is no. But don't 
be put off trying to find some dark matter, it could be useful.

Paul Henderson

[Moderator's note: there is lots more information on dark matter
on our website - try searching at http://www.madsci.org/MS_search.html.]