Gluon

The wavy lines connecting the up quark (u) and down quarks (d) are gluons.

Gluons are what hold quarks together to make bigger particles. Gluons carry the strong force between other quarks, so it is considered a force carrying particle. Photons do the same thing, but for the electromagnetic force. Also, like photons, gluons are spin-1 particles, and when a particle has spin-1 it is considered a boson.

Gluons are hard to study because although they exist in nature all the time, they are so small and require so much energy to break them away from quarks (about 2 trillion degrees) that scientists have only been able to find more about them from particle colliders such as the Large Hadron Collider at CERN.