Being a materialist (as opposed to a dualist, not in the bling-bling sense of the word) one of the topics that come out often in discussions with dualist or dualist-leaning friends is the limits of observation. The argument goes, what if there is some form of matter that you cannot observe?
Well the answer is, there is such matter, which does not interact with the electromagnetic radiation, and thought to be made out of particles that are not included in the current standard model of particle physics.
But their mere existence should be detectable by some means. This means is the gravity that they cause. As a matter of fact their existence were first postulated by observations of the rotational speeds of the galaxies and their distribution in the cosmos, which required more matter than the observed matter within the galaxies.
A new study of the collision of galaxy clusters -nor galaxies but their clusters- provided more evidence on the properties and the existence of dark matter. When the clusters collided, the visible matter slowed down, where as dark matter, sensed through the lensing of light that they cause, did not.
Now it is up to LHC or various different research programs in the world to detect or produce such matter in our galaxy! Wow, what a time to be a physicist!