We went to an interesting lecture last night at Churchill College.
There is a problem with the universe. It doesn't quite behave as we would expect it to given the amount of mass we can see in it. To account for the difference in the observed universe and the what the gravitational models predict in simulations cosmologists have created two cosmic fudge factors, Dark Energy and Dark Matter, which when added to the simulations produce a universe structured much like what we observe today. Together these account for around 95% of the mass-energy of the universe (leaving ~5% for stars, planets, gas clouds and everything else we see, basically all the atoms in the universe). Dr King's talk focused only on the problem of Dark Matter which the models predict makes up ~24% of the universe and left the remaining ~71% of the equation to attributed to dark energy as a problem for someone else to solve.
The problem with observing dark matter is the fact that it is dark. It doesn't emit any radiation and doesn't interact with any of the baryonic matter other than by it's mass effects. The topic of what it actually made of was again left to others to investigate, most probably the particle physics bods at the LHC.
The phenomenon of Gravitational lensing was predicted by Einstein's theory of General Relativity. In fact Arthur Eddington's observation of light bending around the Sun during a solar eclipse was one of the early successful tests of Einstein's theory. In the lecture Dr King talked about using lensing as an indirect detection method for proving the existence of all this invisible mass. Detecting lensing on a galaxy cluster scale (hundreds to thousands of galaxies) is one thing, they don't move that fast relative to the observer and the evidence for dark matter is fairly compelling. However it doesn't really address the problem of where the matter is. We know our galaxy has significant amount of dark matter to hold it together but detecting it with micro-lensing is much harder as things are moving much faster in comparison with each other. The question I never got to ask was how much can we determine about the density of dark matter? Are there clumpy blocks of it or is it spread more homogeneously throughout the galaxy? Does dark matter exist for example in our solar system but at a density too low to detect by it's effects on the local orbital mechanics?
All in all it was a fascinating talk and I certainly learnt more about the topic. Completely un-related to the talk I also learnt that it's now possible to sequence the entire human genome in one week. Go science!