Why do you think there is a fifth force? The four fundamental forces of physics -gravitational, electromagnetic, strong nuclear and weak nuclear - are pretty well understood. But there's always the chance that there's another one we just haven't noticed, perhaps because it's incredibly weak. People have been looking for a new one for a long time. But the forces we're looking for now are different in that they act primarily on dark matter. Just as regular matter consists of a range of particles and forces, I'm motivated by the idea that dark matter is just the lightest, most stable component of an undiscovered "dark sector" of particles and forces.
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