"We can easily forgive a child who is afraid of the dark; the real tragedy of life is when men are afraid of the light." -Plato
Imagine, if you will, the year 2200. Forget about the flying cars and robotic exoskeletons, though. I'm thinking about the incredible scientific tools we'll have at our disposal, as well as the huge set of information we'll have available about the Universe. 
One day, the latest telescope project gets completed, and we're finally able to make detailed measurements of an extra-solar planet's surface!
We'd already been able to learn much about this planet, including its temperature (about 20° warmer than Earth), rotational speed (9 hour days), orbit (approximately circular, 0.82 Astronomical Units in radius), and atmospheric composition. Based on our findings, we had believed that it was teeming with life! Imagine we'd even been able to measure the altitude of various features, including finding sea level (and a sea!), as well as mountains, valleys, plateaus and plains.
But we hadn't been able to image it directly, not until now. So we'd made predictions about what type of life we'd find there. We saw an atmosphere with lots of molecular oxygen, with nitrogen and other elements, much like Earth's. We found carbon signatures on the ground and traces of CO2 in the atmosphere, as well as liquid water and water vapor.
Overall, we expected to find life similar to life on Earth, except, with this new planet being hotter, we expected more of the tropical-style plant and animal life. What did the new telescope reveal?
For some reason, this planet, although covered with its own amazing biodiversity, didn't have any angiosperms on it. No leafy, deciduous trees, no flowering plants, nothing, in fact, resembling the lush, forested land our best astrobiologists had expected. The flora, surprisingly, resembled pine trees more than anything else, even in the warmest, wettest, most tropical climates.
But, despite what was immediately dubbed "the angiosperm problem," scientists hailed this as a triumph. With a successful prediction of life, many stages of its evolution, and a large number of specialized adaptations, the astrobiologists were almost completely satisfied.
But there were a small number who were convinced that they had it all wrong. Where were the flowers? Where were the maples? Where were the fruit-bearing trees? Standard Astro-Darwinian theory dictated that they ought to be there! So they created their own, rival theory, Extended Gymnospermic Astro-Darwinism, or EGAD! It still predicted life, and it also predicted gymnosperms outcompeting angiosperms. But it failed to predict many of the things that standard theory predicted, giving incorrect predictions for the adaptations to temperature and altitude, as well as failing to predict the evolutionary timeline correctly. Still, a few scientists could not get past the angiosperm problem, and rather than trying to solve it, abandoned all of the accepted astrobiology in favor of working on EGAD.
This story, of course, is pure fiction, and since I wrote it, there's probably a whole slew of things wrong with the biological assumptions in it.
But right now, in my field of cosmology, the same thing is going on with the dark matter problem. Although I've already told you what convinced me that dark matter exists, I'll briefly recap some of what dark matter explains.
It successfully explains the observed large-scale structure of the Universe. Simulations without dark matter all fail to match the observations, but with dark matter, they match up practically perfectly.
Look at the individual galaxies within any of the many galaxy clusters we see, like the Coma Cluster, above. They're all moving too quickly to be held together by just the gravity from protons, neutrons, and electrons. An extra, dark type of matter must be present in this cluster. Why am I so convinced that it isn't more protons, neutrons, and electrons? Or that the law of gravity just isn't different than we expect?
Because when we look at two colliding clusters, above, we see that gravity (in blue) doesn't line up with where the normal matter is (in pink)!
But that's not all. Without dark matter, there would be a whole mess of things that didn't work out.
The matter power spectrum of the Universe, for one. (Image from Eisenstein & Hu, 1998.) The top sets, with all of the matter being normal matter (and irrespective of your law of Universal Gravitation), is completely ruled out. But if you allow some of that matter to be dark matter (i.e., not protons, neutrons, and electrons), you get a graph that looks more like the bottom set, which is also what we see in the Universe.
The spectrum of fluctuations in the microwave background is another one! If you look at what you get (data points, above), and compare it with predictions involving dark matter (solid line, above), it lines up pretty well! But if you take the dark matter away, the shape of your curve looks like this.
I know, because I calculated it myself! I'll just give you one more.
Without dark matter, you'd need much more normal matter in the Universe in order to make the Universe the way it is. But you can't have any more than we see, because you would screw up big bang nucleosynthesis! The abundance of the light elements is well observed, and consistent with there only being about 6 nucleons (protons and neutrons combined) for every 10 billion photons; try to put more in there and you'll conflict with what we see.
Bottom line, dark matter -- this one addition -- makes these and a whole host of other observations work out. But, much like the fictitious astrobiologists and their predictions of angiosperms, there's one thing that dark matter gets slightly wrong.
For individual galaxies, dark matter predicts a huge, diffuse, massive halo surrounding the galaxy. This should have a gravitational effect, and should cause the galaxy's rotational speed to vary with distance according to either the curve marked "Moore" or "NFW", below.
But it doesn't! It matches the line marked "isothermal." In other words, dark matter is wrong in the details it predicts for individual, galaxy-scale objects. And this is a genuine problem for the theory of dark matter. It's a puzzle that most of us are hoping to work out.
And, as you might expect, just like there were the EGAD group of scientists in the story at the top, there are people in favor of modifying Newton's Laws of Gravity to explain these galaxy-scale features. If you do Modify Newtonian Dynamics (the theory of MOND), you can explain the galactic-scale observations better than dark matter can. But you can't explain any of the others.
Any of them. And the predictions that even simple models make are wildly in conflict with a huge number of observations. So, most of us try to fix the problems with dark matter on galactic scales by studying it better, and trying to figure out what's going on with the halo.
But I am compelled to tell you all of this today. Why? Because over in Germany, one of the most media-visible scientists, Pavel Kroupa, has been harping on the galactic-scale successes of MOND, and touting it as a superior alternative to dark matter. In addition to a number of press releases, he's even started his own blog on the SciLogs network, The Dark Matter Crisis.
His writing is focused on this one issue in many different incarnations: the incorrect predictions of dark matter halos, in the detail of their density profiles, on scales of a single galaxy and smaller.
I'm not saying that there isn't something to be learned from MOND, but I am saying that it isn't a better explanation for anything except the dynamics of an individual galaxy than dark matter. But it was designed to explain the dynamics of an individual galaxy. Until you manage to accomplish something else with MOND, I'm not only going to support dark matter, I'm going to keep you honest about your claims of what MOND does and dark matter doesn't do. It's important to consider alternatives to the standard theories, to be sure, but not to do it at the expense of all of your evidence.
Don't ignore the great forest of evidence, on all of its different scales, just because the individual trees look different than you were expecting. Try to figure out why the trees are different, and try to reconcile that with the fact that they're definitely found in a forest.
