I am a string theory kind of person!

I don't pretend to know. Neither seems more or less plausible to me.

1) Visible things explain only a little part of our observations. Visible light is only a little part of the spectrum. And gravitation, for instance, is not visible, just measurable. So one could say that Newton invited something invisible, gravitation, to explain the world... Black holes, too, are invisible. We could just observe things disappearing as they approach a black hole. 2) " Vera Rubin, a young astronomer at the Department of Terrestrial Magnetism at the Carnegie Institution of Washington presented findings based on a new sensitive spectrograph that could measure the velocity curve of edge-on spiral galaxies to a greater degree of accuracy than had ever before been achieved. Together with fellow staff-member Kent Ford, Rubin announced at a 1975 meeting of the American Astronomical Society the astonishing discovery that most stars in spiral galaxies orbit at roughly the same speed, which implied that their mass densities were uniform well beyond the locations with most of the stars (the galactic bulge). This result suggests that either Newtonian gravity does not apply universally or that, conservatively, upwards of 50% of the mass of galaxies was contained in the relatively dark galactic halo. Met with skepticism, Rubin insisted that the observations were correct. Eventually other astronomers began to corroborate her work and it soon became well-established that most galaxies were in fact dominated by "dark matter"; exceptions appeared to be galaxies with mass-to-light ratios close to that of stars. " Source and further information: http://en.wikipedia.org/wiki/Dark_matter#Galactic_rotation_curves 3) So there was an observation, and "either Newtonian gravity does not apply universally or that, conservatively, upwards of 50% of the mass of galaxies was contained in the relatively dark galactic halo" In such a case, the scientists use to take the most conservative theory. To modify Newtonian theories would have been more complicated, it seems. But these alternative explanations still exist: (Modifications of gravity, Quantum mechanical explanations) Further information: http://en.wikipedia.org/wiki/Dark_matter#Alternative_explanations 4) "Recently (2005), astronomers from Cardiff University claim to have discovered a galaxy made almost entirely of dark matter, 50 million light years away in the Virgo Cluster, which was named VIRGOHI21. [...] Based on rotation profiles, the scientists estimate that this object contains approximately 1000 times more dark matter than hydrogen and has a total mass of about 1/10th that of the Milky Way Galaxy we live in. For comparison, the Milky Way is believed to have roughly 10 times as much dark matter as ordinary matter. Models of the Big Bang and structure formation have suggested that such dark galaxies should be very common in the universe, but none have previously been detected. If the existence of this dark galaxy is confirmed, it provides strong evidence for the theory of galaxy formation and poses problems for alternative explanations of dark matter." Source and further information: http://en.wikipedia.org/wiki/Dark_matter#Velocity_dispersions_of_galaxies So the Dark Matter Theory could have the strongest evidence at the moment. 5) I would rather change the rules than create something invisible to explain the world. But of course, in some cases (such as DMT) it can be very justified to assume the existence of something invisible. This existence is also predicted by the very popular Big Bang Theory. 6) And there are also other "invisible" things which we use to explain the world, for instance thoughts and feelings. And sometimes angels.

I recently had to do a talk on DM and MOND. Although MOND is an interesting and investigates the other side of the coin. That is that to get what you see in the Universe either you need more mass (Dark Matter) or you need the laws to change under certain conditions (MOND). Although DM itself was ridiuled until the late 1970's it has about 20 years of refinement over the MOND camp which unbalances things a litte. I think MOND is a nice theory and tests DM in places that people wouldn't have thought of, so makes DM theories more verifiable. Unfortunately MOND is just too free-floating, if something goes wrong in that an observation doesn't match your MOND predictions you can just alter MOND to make it fit. This is somewhat of a boon and a curse and as such there are few variables you can tie DM down with. Also some recent centre of mass analysis of the Bullet Cluster has caused problems for MOND (in that it supported dark matter not mond) and the answer was for MOND to make its own form of invisible matter. This just makes MOND less believable in that it was purely invented by Milgrom in the 80's to remove the need for dark matter and here it crops up again some twenty plus years later (in one form or another). Though some other MOND variants like MOG say that they don't need this invisible form, this just highlights the easy variability of MOND and the fact that observations can't constrain it's variables very easily.

I am undecided although I tend more towards Dark Matter etc. I don't like changing rules in mid stream, it makes me deeply uncomfortable. It reeks of unfalsifiability whereas DM is measurable (maybe not visible and difficult but certainly measurable and we'll get better!).