The Standard Model cannot explain why a particle has a certain mass.
For example, both the photon and the W particle are force carrier particles:
why is the photon massless and the W particle massive?
Physicists have theorized the existence of the so-called
Higgs field, which in theory interacts with other particles to
give them mass. The Higgs field requires a particle, the
Higgs boson. The Higgs boson has not been observed,
but physicists are looking for it with great enthusiasm.