Scientists call the everyday matter of our world, such as tables and chairs, “ordinary matter.” Ordinary matter is made up of atoms, which in turn, are each made up of a nucleus and electrons.
Antimatter. Antimatter is formed of particles which are identical to electrons and protons except that they are oppositely charged. In other words, antimatter has electron-type particles which have a positive charge and proton-type particles which have a negative charge. All subatomic particles of ordinary matter are believed to have antimatter counterparts. Scientists rarely encounter antimatter, but there may be large amounts of it at greater distances from Earth than we can currently observe.
Dark matter. Dark matter is a name that’s been given to a substance which many scientists believe may exist. If it exists, this substance could account for the amount of gravity that astronomers have observed in galaxies. Astronomers believe that more gravity is at work in galaxies than could be generated by the ordinary matter that they observe, that is, the number of stars and other heavenly bodies that they can see through telescopes or radio telescopes. Their theories tell them that there is “missing mass” in the galaxies that they observe.
One possibility is that the missing mass is present but invisible to us for some reason. If it’s present, it’s not believed to interact with light or other electromagnetic waves, like radio waves. That would account for why it can’t be seen through telescopes or radio telescopes. As they can’t see it, astronomers call what they believe may be hidden, “dark matter.”