Is there a "fifth force" beyond the four we currently know of?
The world as we know it can be described by four forces: gravity, electromagnetism, and the strong and weak nuclear forces. In the language of particle physics, each of those forces is "mediated" by an associated particle: the graviton for gravity, the photon for electromagnetism, the gluon for the strong force, and the W/Z bosons for the weak force. But the existence of dark matter suggests that there may be additional forces responsible for mediating interactions between dark matter particles. If we're lucky, these same mediators may interact (very weakly!) with visible particles, making it possible to detect dark matter.
One possibility for this "fifth force" is a particle we nickname the dark photon. "Dark," because it interacts mostly with dark matter, and "photon" because its interactions with charged particles are just like the interactions of the ordinary photon, but much weaker. Unlike the massless photon, the dark photon can be massive, which leads to distinctive experimental signatures: for example, a dark photon radiated from a high-speed electron wants to steal all of the electron's energy, while an ordinary photon would prefer to sneak away with as little energy as possible. My research is focused on designing novel experiments to detect such dark photons and their interactions with dark and visible matter, and on using a dark photon to explain persistent anomalies in precision low-energy physics.