Dark matter is the modern fairy dust that makes everything (cosmologically) better. Upon observing that the universe would need to have several hundred times more mass than we can see to be consistent with modern theories, many scientists apparently thought "Fair enough, there must be several universes worth of invisible magic stuff hiding throughout the entirety of existence". Rather than the crazy heresy of "Maybe our theories need a little work".
Lots of very smart people have come up with a huge variety of explanations for this absentee material, from ultrarelativistic non-baryonic matter (yes those are real words, even though it sounds like they just smashed together some sciencey sounding stuff), to effects from other universes. The latter is supported by brane theory, but when you're combining both dark matter and superstring theory you might as well throw in "A wizard did it" to explain all the flaws.
Normally people who constantly look for fictional things are labeled as crazy, and this scientific search may have reached psychosis point with spiral galaxy NGC 4736. A team from the Polish Academy of Science have observed a spiral galaxy that doesn't need the dark stuff. This galaxy can be entirely explained in terms of the matter we see and the theories we have, and in a shocking reversal of the whole "Scientific Method" process other researchers are criticizing the find - because the observations don't support the invisible thing they believe in. That's not science, that's religion.
Another suspect for this maybe-missing-matter is the neutralino, and we'll have to ask you to believe us that we're not just making up words at this point. As a heavy, stable, weakly interacting particle it has all the right properties to hang back and just "be there" in the invisible way dark matter is expected to be, with only the usual slight flaw of being an utterly hypothetical string symmetric construction - but maybe not for long. NASA's gamma gazing GLAST satellite is planning to survey a radiation map of the sky for comparison with neutralino predictions, meaning it will be testing for dark matter, string theory and supersymmetry. No word yet on whether it's fitted with a unicorn detector too.
Posted by Luke McKinney.
Not seeing what isn't there http://space.newscientist.com/article/dn13280-galaxy-without-dark-matter-puzzles-astronomers.html
GLAST going for gamma rays http://space.newscientist.com/article.ns?id=dn11743&feedId=space_rss20