Dark Matter vs. Alternative Theories

[suburban]

Hi everyone,

alright my question is this, what are the reasons for such a strong and widespread belief that some dark, as of yet unobserved type of matter must be present in the universe?
My knowledge of the topic is rather superficial but the way I reason is this, there is mathematically and physically appealing theory called general theory of relativity and for quite some time this theory has been repeatedly tested (with positive outcomes). The precision of the tests is nothing like the experiments in quantum mechanics, but hey, GTR always gave better predictions than any other theories and up to date. I think that only Brans-Dicke theory and some of the f( R) theories can match up to GTR (with a whole lot more complications and with a few free parameters that in the end can always be chosen to make the theory as close as it needs to be to GTR).

Taking only this into account it would seem that if something doesn't work the way it should according to GTR then we're missing something out there, but the important point that we didn't take into account (at least it seems that way to me) is that GTR has been very well tested in our solar system and here it gives quite remarkable predictions. Once we make observations of galaxies and they don't follow the rules set by GTR why do we insist GTR is right and our eyes (alright, our big-diameter wide-bandwidth electronic eyes) are wrong ?

Can't it just be that, even though in our system and in our Sun's gravitational field the theory is a good effective theory, on the large scale of galaxies it is just fundamentally plain wrong ?

To summarise, my question would be, how do we know that GTR is stil true on large scales when it fails to predict correct motion on both galaxy and universe scales (we must add additional ingredients to make it work = doesn't work) ? And second what are the possible options for modified theories of gravity ?

 

Thanks folks!

Edited February 20, 2013 by suburban

[Arch2008]

You may want to wait two weeks, because Dr. Lykken may or may not have found the decay paths of dark matter with the AMS. He is supposed to publish a paper about this.

[swansont]

I recommend you read Ethan Siegel's blog, Starts With a Bang. He's a cosmologist and writes on the subject quite a bit. Here are two recent posts that are on topic

 

http://scienceblogs.com/startswithabang/2012/12/07/yes-we-really-really-need-dark-matter/

http://scienceblogs.com/startswithabang/2013/01/18/why-the-universe-needs-dark-matter-and-not-mond-in-one-graph/

 

Short version: whatever issues you might have with the concept of dark matter, alternative theories are worse.

[suburban]

so i guess it comes down to this picture (from one of the articles that you linked):

 

where red squares are data, dotted blue line is GTR without dark matter, continuous blue line is one of the alternative theories (here to represent all of them) specifically relativistic extension of MOND, and finally black line is the good old Einstein + dark matter...

 

it's actually as convincing as it gets... thanks !

so now I guess the question remains of how bad do other theories complicate things and is there really no some fundamental principle on which to base our eq's to get something as "compact" as Einsteins theory ?