I recently came across a page explaining that dark matter exists. In case you don’t know, dark matter is this mysterious stuff interacting very weakly, if at all, with normal matter, except for a gravitational effect. Dark matter is one way to explain why galaxies do not move the way they should as predicted by general relativity. An additional hypothesis, dark energy, with even more exotic properties, is required to explain the observed cosmic expansion. The real problem with all this dark stuff is that it would have to make up up 95% of the grand total. That’s right: regular matter and such would not amount to more than about 5%
The article above does a pretty good job at explaining the origin of these ideas, and furthermore, gives a rather convincing argument that dark matter has been observed. The general idea is that the X-ray picture of the “bullet cluster”, a place where two galaxy clusters collided, does not superimpose with the gravitational field in that region. This is clearly visible if you superimpose the two.
Now, to appreciate how much work goes in such research, you have to realize just how difficult it is to evaluate the gravitation field at such distance. In this case, they use weak gravitational lensing, an application of Einstein’s prediction that gravitation bends light.
Ultimately, however, I think that the conclusion “Dark matter exists” is just a little bit premature. We know of so many things that bend light (beyond gravitation, think of a simple aquarium or mirages) that assuming that minute light bending in an area that just went through a massive collision betwen two bodies has to be the result of gravitation alone seems a bit far-fetched to me. If hot air can bend light, couldn’t “hot” interstellar gas bend light as well?
Comments are closed on the article above, otherwise, I would have asked the question. It’s possible that there is a very good reason to rule out any explanation based on a change in the large scale refraction index. But I don’t see that mentioned anywhere.