We were following a methodology where you put forward a candidate with the goal of ruling it out in order to narrow down the possibilities. With cold dark matter we failed miserably in that sense. We haven’t been able to rule it out. In all the calculations that we did and the many follow-ups people have done that have extended our work, they all come back to the same thing: cold dark matter universes look a lot like the real thing.
The fact that cold dark matter looks so good in a computer simulation doesn’t prove, of course, that it is the force shaping the universe. Today it is the front runner candidate, but until we actually see a WIMP, we can’t be sure. There are other possibilities that need to be explored and those can be explored within the context of computer simulations.
The key point of these theories is they require the existence of these hypothetical elementary particles. The proof of the pudding is in the eating; you have to capture one of these particles. So the ultimate test of this cold dark matter theory is to find the cold dark matter directly. Physics is, after all, an empirical, experimentally based human activity. You can’t prove that something is correct by theory. The Greeks thought that the truth could be established by pure thought, but we now know better: the universe is not made that way. We cannot prove the reality of anything just by thinking about it.
It’s hard to prove these particles exist because they’re very weakly interactive; that’s why they are dark: they don’t interact with anything. Cold dark matter doesn’t experience electromagnetic or nuclear interactions like protons and electrons do. They don’t interact with your apparatus, so trying to detect them in the laboratory is like trying to catch water using a bucket full of holes; it just goes through it.
Still, there are experiments to detect even these very weakly interacting particles by side effects. If you have a semiconductor, occasionally one of the WIMPs could have a head-on collision with a silicon atom and cause the atom to recoil . Now these hits are very, very rare so you have to have several kilograms of semiconductor and you are trying to find one atom moving just because it gets hit by a WIMP. Until these experimental searches succeed we cannot be certain that the theories are correct. But the exciting part is that the experiments are in place and the particles are detectable. If they exist, we will know about them in a few years.