It is one of the great embarrassments of modern physics – the overwhelming literature on dark matter. A quick search through the arXiv (the repository for academic papers) reveals thousands of papers generated every year on the nature of dark matter, and even limiting it to serious contenders still provides a list of over 100 possible forms that dark matter could take.

When physics is focused on finding the one correct model, how could it happen that we have spent decades studying dark matter and not come close to knowing what it is. It isn't exactly rare – it forms roughly a quarter of the energy in the Universe and close to 90% of the matter. It would be more accurate to say that 'normal matter' (like the atoms the make up everything we know) is the rare stuff. So why can't we pin down the nature of dark matter?

The answer seems to come from what we do know about dark matter. We know from studies of the cosmic microwave background that it has mass and interacts through gravitational forces. We also know that it does not interact with light or any other electromagnetic fields (or at least so weakly as to be undetectable). From experiments at the LEP collider we know that dark matter does not interact significantly through the weak nuclear force, and data from various colliders restricts how much it can interact through strong nuclear forces. So we have ruled out the three forces of nature that can be tested easily in the laboratory.

There are many other experiments that are seeking dark matter. Experiments like DAMA and CDMS use various elements in either liquid or crystal form to detect dark matter particles from space, by carefully measuring any recoils when the atoms in the detector collide with an unknown particle. However as yet they have only been able to limit the properties of dark matter, and have yet to find a clear signal of a collision. There are also particle accelerators like the LHC or the Tevatron that have been trying to produce dark matter particles from the high energy collisions of other particles, but as yet they have also detected nothing.

And unfortunately both nature and theoretical physicists abhor a vacuum. Because there is very little data on the nature of dark matter, theorists can imagine a wide variety of particles that are consistent with the data. Maybe it is a byproduct of supersymmetry, or extra dimensions, or an as yet unknown fifth force. Maybe it is simply a particle that is unconnected to any other theories and just lives in isolation. Maybe we don't really understand gravitational forces and it is an illusion caused by some new aspect of gravity. Or maybe it is something so exotic we cannot even imagine it yet.

For the present, all that we can do is wait and see what the experimentalists find. And in the meantime we can enjoy speculating!