Today, my 16-year old son asked me what dark matter was. I was surprised that he would even have heard about dark matter, but it turns out that even junior science magazines talk about the search for dark matter these days. I must say that I'm not too happy about that. The junior science article, like many other, present dark matter practically as a fact.
The reason this makes me rather nervous is because of the rather obvious parallel with aether. Just like the luminiferous aether, dark matter is something that was postulated when no physical evidence justified it, in order to preserve existing theory.
Those of you who were already dabbling in physics during the 1850s1 may recall that luminiferous aether was hardly a ridiculous idea at the time. Aether was very simply the medium carrying light waves, much like air or water carry sound waves. It was initially postulated by Isaac Newton to explain things like refraction. According to Wikipedia, Augustin Fresnel proposed in 1818 a theory of light as a transverse wave in aether. To quote Wikipedia, from this point on, no one even seems to question its existence. In other words, the existence of aether was postulated in order to preserve the existing theory of waves. All existing waves required a medium, such as air or water, therefore it was natural to assume that light waves also needed a medium to carry them.
The key point to remember here is that the brightest minds of the time did not question aether at all. Some of them, like Newton or Fresnel invented it. Later, the vast majority of scientists were busy trying to refine the concept to make it work. Yet today, luminiferous aether is seen as the canonical example of an obsolete physics theory. Einstein's relativity made the very notion of aether not just useless, but actually wrong. Relativity simplified things by removing the need for a system of coordinates that would be special, but this simplification meant that aether could not exist, because otherwise aether itself would have defined a system of coordinates that was unique.
Back to dark matter. We find ourselves in a similar situation today. There's something about the universe that we very plainly, very visibly do not understand. The original problem, as identified by Fritz Zwicky, was that galaxies do not spin the way they should according to our best theory of gravitation, general relativity. They behave as if there was more matter in them than we can see.
The operational keyword here is as if. At the moment, we really have no idea whether it's the theory of gravitation that is flawed, or whether there really is 95% of the universe's mass that we can't detect. Talking about "dark matter" is choosing one option over the other. It's pretending that we know, when in reality we still lack a model that really explains all the evidence. In my humble opinion, the jury is still out on what this model will look like.
In short, I'm unhappy about references to dark matter made as if it was a settled topic, a known, validated scientific fact on a par with photons or Pluto. Maybe the problem is with the terminology. Talking about dark matter rather than, say, "gravitational anomaly in galaxies" (GAG) is a good way to preserve the illusion that we know what we are talking about. It makes it sound real. But just because we gave it a fancy name doesn't make it more real than aether or the tooth fairy.
Let's be humble and honestly face the simple fact that our model of mass and gravitation breaks down in face of quite a bit of physical evidence. We find ourselves in the situation of physicists in 1850 whose aether-based theories predicted phenomena like aether drag and aether wind, which experiments repeatedly didn't find. It's exciting, it's fun. It's a good thing for physics, because it means there is something new to be found.
Note 1: My editor tells me it's considered bad taste to live past 150 on this planet. My apologies to those of my readers I might have offended...