Something is missing. Most of the universe, actually.
The matter and energy that we know about only seems to account for 4% of the observable universe. What the rest is made of is a bit of a mystery. It clearly hasn't just fallen down the back of the settee. There must be something else�
The existence of dark matter is inferred from comparing what we see of the universe against what we would expect to see from our understanding of how gravity works. It may turn out that we've got completely the wrong end of the stick about gravity, and that it acts over galactic distances differently to how it acts on the scale of, say, the solar system. More likely, there is some form of matter that accounts for the vast majority of the stuff the universe is made of, which does not emit or reflect radiation that we can perceive - "dark matter". Without dark matter, the numbers just don't add up.
The evidence for the existence of dark matter is pretty compelling. Think of a typical spiral galaxy, like the Milky Way, spinning away happily in space. You'd expect stars near the middle of it to spin faster than the outside, just like Mercury orbits the Sun in 88 days, while a year on Neptune lasts 165 Earth years. This just isn't what astronomers are seeing. Galaxies are spinning as if the stars in them are dots painted on a spinning plate, with the outermost stars moving faster than they have any right to. The only conclusion, if our concept of gravity is right, is that there's loads of invisible matter in galaxies holding the whole show together. We're seeing the dots, but not the plate.
There is other evidence as well, like whole groups of galaxies that rotate as if they were embedded inside an invisible ball. And then there's the distribution of cosmic background radiation, the after-glow of the Big Bang, which strongly hints at a lot more matter in the universe than anything we can yet perceive. And there is more: using a technique called gravitational lensing, astronomers have taken a stab at mapping the distribution of dark matter in the observable universe.
All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contains far more matter than that which interacts with electromagnetic radiation.
Dark matter is there.
So what could it be? There are loads of theories. Clouds of non-luminous gas and ghostly particles like neutrinos could explain some of it. Massive, invisible, hard-to-detect astronomical bodies like neutron stars and black holes (collectively called MACHOS) could also account for some, but most dark matter is probably a completely unfamiliar form of "non-baryonic" matter that isn't made of protons, neutrons and electrons at all.
The smart money is on a shadow world of undetected particles, called supersymmetric particles (or sparticles). The idea is that when the fundamental particles we know about were produced in the Big Bang (like electrons, photons, and quarks), each was accompanied by a matching sparticle. These hypothetical partner particles have some cool names like photinos, selectrons, squarks, gluinos, and neutralinos. There should even be Higgsinos and gravitinos - the hypothetical counterparts of hypothetical particles!
It sounds a bit mad, but the existence of sparticles actually ties in neatly with the best model we have of fundamental particles and their interactions - the Standard Model. It does mean doubling the number of particles in the model at a stroke, which seems a bit drastic, but if that's where the evidence is pointing we have to go with it�
The neutralino, a heavy, stable, neutral partner particle of the neutrino that barely interacts with other particles, is top of the list of suspects for dark matter. They call it a weakly interacting massive particle: a WIMP.
But do sparticles exist? If so, they'd have masses hundreds of times greater than their "normal" counterparts, so it will take an accelerator with lots of horsepower to reveal them. Remember, the more mass a particle has, the greater the collision energy needed to create it. It is completely possible that the LHC experiments will conjure these elusive sparticles into existence, so that we can "see" them. If this happens, it won't just resolve the question of dark matter. It will usher in a whole new kind of physics�