First off, i'd like to thank everyone reading this page for their encouraging comments, I just hit 50 followers recently and I hope that the information I try to break apart here is of some use to you. If it's not, or if you interests lie in a different direction please let me know! I am always open to talking about specific topics of interest, if there is anything you feel you would like me to elaborate on or attempt to convey in an understandable manner just leave me a comment and ill do my best to accomodate.
Second, Comcast sucks, I don't know about the rest of you but I have always had problems with them trying to stayconnected when I need to, otherwise this would have gone up much earlier today.
Today I want to talk a little more about this stuff that scientists have dubbed "dark matter" and I think the best place to start is why we would even think it's there in the first place. I mentioned last time about how astronomers use gravity to notice something that should be there when you can't detect it normalls, but the manner in which they do that is much more refined. Over extremely great distances everything is affected by gravity, even light, so one of the primary methods for detecting this dark matter is known as "gravitational lensing". When light from a very bright object in the sky (even brighter than most individual galaxies we are talking about exploding stars and large clusters of galaxies here) is distorted (or bent or however you want to say it) into arcs or curves then it's possible to measure the angle of this distortion and find out how large this invisible object is (or at least it's mass, which doesnt really tell us alot about it's physical size. You can see an example of lensing below.
There are a few other methods for detecting the presence of dark matter in the sky, including looking at the data provided for the CMBR (see earlier post) however let's talk about what we actually know about this stuff, this matter that we can't see. Most of the information about what dark matter is actually composed of is speculative (or nobody really knows but it's fun to make stuff up) and most of them revolve around large mass objects that we wouldn't be able to see normally: black holes, small faint stars, and some kinds of gas are all thought to be contributors.
Some have postulated that there are at least three forms of dark matter, cold warm and hot, however these terms deal more with their speeds than their temperature. Cold dark matter is the stuff that travels at speed we are used to, known as "classical" speeds. Warm moves at nearly the speed of light and so faces interaction with the doppler effect mentioned in an earlier post. Hot dark matter is thought to move at speeds extremely close to the speed of light, so close most people would just round up. The cold dark matter represents the greatest interest to modern scientists because of it's relatively "slow" speeds it would be a large factor in how galxies and their clusters were formed towards the beginning of the universe.
In any case attemting to directly detect dark matter is pretty frustrating. Most use one or two technologies to detect this stuff, and have to be situated deep within the earth to protect it from interference from all of the other radiation raining down on our planet at any given moment. One popular method is a detector so cold it reaches down near something dubbed "absoulte zero" if we are talking about celcius it's around -273, and detect the slight variation in temperatures. Needless to say keeping a detector that cold all the time is pretty expensive.
To summarize, dark matter is simply a term that reflects the modern scientific world's ignorance of what nearly 25% of all the detectable material in the universe is composed of. Detectable only through indirect methods, it will be some time before our techniques are advanced enough to really understand what's going on out there.
Next time I think we will go into the speed of light and why it's so important that nothing can ever pass it (think of it like a universal speed limit).