A plasma is a gas of ionized (i.e., electrically charged) particles. It has distinctly different properties than a gas of neutral particles because it is electrically conductive, and responds strongly to electromagnetic fields. Plasmas are typically either very hot or very diffuse because in a cool, relatively dense gas the positively and negatively charged particles will bind into electrically neutral units. The early universe is thought to have passed through a stage in which it was a plasma of quarks and gluons, and then a stage in which it was a plasma of free protons and electrons. The electron gas inside a conductor is another example of a plasma. The intergalactic medium is an example of a cold, diffuse plasma. It is possible to create an ultracold plasma using the techniques of atom cooling and trapping.