Getting to the answers to the questions posed in the title of this stimulating lecture proved quite a challenge for our speaker from Nottingham University, Dr Julian Onions – and not for the want of trying but because the answers are proving rather elusive. To tackle the issue and give us some background, he asked three further questions – why do we believe it exists, what could it be and how could we find it?
It is currently believed that the Universe consists of Atoms (4.6%), Dark Energy (72%) and Dark Matter (23%). In much the same way that the postulations of the Neutrino and Higgs Boson well pre-dated their actual discoveries, so too has the notion of Dark Matter, first proposed in the 1930’s by Fritz Zwicky following his studies of the motions of galaxies and the amount of light emitted in the Coma Cluster. The concept of Dark Matter was required to account for the observation that the gravitational mass of the galaxies within the cluster was at least four hundred times greater than expected from their luminosity. Later modelling studies of galaxy formation also didn’t reflect what was going on without dark matter being part of the models. Yet further evidence for its existence came from work on Gravitational Lensing and studies of the Cosmic Microwave Background.
Quite what it could it be, Dr Onions continued, brought us candidates such as axions, sterile neutrinos, weakly interacting massive particles (WIMPs), gravitationally-interacting massive particles (GIMPs), super-symmetric particles, and primordial black holes. Not to forget MACHO’s (Massive Compact Halo Objects)! But right now there is no technology available to detect dark matter – and the cost of it makes the price of the Large Hadron Collider and the LIGO detectors seem just like petty cash!
So what conclusions can be drawn?
Is it Cold? – Maybe, but it might be warmish.
Is it Dark? – Also maybe, but it could just be transparent.
Is it Matter? – Mmm, not really.
And in the end, does it matter? Well, probably yes! Because if we discard it as a concept, we might need to completely rewrite our understanding of physics and quantum mechanics … and gravity … and relativity … and cosmology ….