Matter is the stuff that makes up our universe. It has mass, and occupies space. All physical objects, materials and substances are composed of matter in one form or another. You are no doubt familiar with the three most common states (sometimes called phases) in which matter can be found - solids, liquids and gases. We will also be looking at a fourth state of matter - also relatively common - called plasma.
In addition to these four states of matter, there are a number of more "exotic" states - some of which have been proven to exist, and others that are as yet only theoretical. These exotic states of matter occur (or in some cases are thought to occur) under extreme circumstances. Matter can behave in unexpected ways, for example, under extreme pressure, or at extreme temperatures. For the purposes of this discussion, we will confine ourselves to the four common states of matter.