The bottom line is, nobody really knows
what dark matter is. Many (a majority) of classical physicists believe it exists, because it's existance is required by the classical theories. Since it fills a "hole" in the theory, they can deduce some of it's properties based on the phenomena it has to account for, but this isn't the same as knowing what it really *is* or even proving that it exists.
Other physicists see the failing of classical physics to fully explain things and point to the same hole and come up with even more radical theories to explain it, and usually also attempt to make these new theories reconcile the apparent incompatibility between the relativistic and the quantum level understanding of the world we presently have. The most popular such alternative theory is the various forms of String Theory, but to date string theory isn't an agreed-upon, cohesive set of theories or formula like classical theory is. Lots of people are hopeful that string theory is "thinking in the right direction" but few would claim that it can explain as many phenomenon as accurately as classical theory, at least so FAR.
Ultimately, tho, while most physicsts believe in dark matter, they don't agree at all on what it IS, and while they all have theories and guesses, nobody to date KNOWS what dark matter is.
So, who's to say you couldn't compress it into ultra-dense balls inside a bizarre alien life-form, or liquify it and greatly inconvenience a whole bunch of penguins? Informed people might dismiss it as silly, but they can't *prove* it's completely impossible.
Oh, and one last comment, "dark matter" and "antimatter" are usually only put forward as a way to produce enough ENERGY to power an FTL spaceship drive, with some other even more fanciful explanation for how that drive itself works. Farnsworth and cubert's explanations of the ship going ftl didn't talk about dark matter, and on Star Trek, it's the warp drives that let them exceed light-speed, which the federation happens to power with antimatter reactors.
I'm a science dork AND a science-fiction dork.