The smallest particle is the quark, which forms protons and neutrons. There are six different types of quarks, called "flavors" by physicists. They all have different masses, but scientists group them in pairs. The two lightest quarks are "up" and "down" quarks that, combined, form protons and neutrons. The two other pairs are the "charm" and "strange" pair and the "top" and" bottom" pair. The last two heavier pairs of quarks don't exist in nature, as scientists proved their existence in particle accelerators.
All atoms have gluons that keep their nucleus together. Gluons are force-mediating particles that have stronger nuclear force than any other known natural force. The force is 137 times stronger than electromagnetism and about 1.6 x 1039 times stronger than gravity. The force of gluons holds together all known matter, except dark matter, which still isn't researched enough. The limitation of gluon force is it operates at extremely small distances. If the distance becomes longer than the width of a medium-size atomic nucleus, it begins to fade.
Just moments after the Big Bang, the space was filled with plasma so hot and dense that no nuclear particles could exist. This plasma consisted of quarks and gluons. Quarks and gluon interact in a different way than any other particles. If the plasma is exposed to low temperatures, it will become denser and the attraction force between quarks and gluons will be stronger. Because of this, free quarks are not found in nature.
Scientists around the world are trying to "turn back time" and explain what matter looked like at the start of the universe. The first machine built for this purpose was the Relativistic Heavy Ion Collider (RHIC). The RHIC uses beams of gold ions and collides them at the moment when they travel at nearly the speed of light. The collision "melts" the protons and neutrons and frees quarks and gluons. In this way scientists can examine the collision area and find significant information about the way the universe cooled and expanded since its beginning.