The Big Four
The present understand of physicists and astronomers is that all interactions between matter are mediated by only four forces - and in fact two of these forces (electromagnetism and the weak nuclear force) have already been "unified" into a more general force which describes both as different sides of the same coin. Hopes for a quick unification of a third, strong nuclear force with these two - a "Grand Unified Theory" were dashed in the early 1980's when the IMB experiment and Kamiokande failed to find the signal of proton decay predicted by the favored candidate for this theory failed to materialize. Super-Kamiokande continues to search for proton decay, the discovery of which could truly spark a revolution in physics. A somewhat different approach termed "Supersymmetry" is now the preferred point of departure among theorists. Discovery of the even one of the many new particles needed to make Supersymmetry a reality, either at Fermilab or the future LHC machine at CERN would shake the world of physics to its foundations. The fourth force, gravity remains resistant to theoretical unification with the other three, since the quantum effects which must play a role in any such theory are far beyond the reach of contemporary experiments. Gravity, inextricably bound up with the question of mass, is ironically one of the main reasons for excitement over the announcement of discovery of neutrino mass.